Full volume in the library

• Original cast members of The Cheviot, the Stag and the Black, Black Oil, John Bett, Issie MacPhail, Elizabeth MacLennan, Bob Tait, Dolina MacLennan and David MacLennan, in front of the pop-up book set. Picture: Jane Barlow

It's a reunion of old friends and colleagues, certainly; it's 37 years since this group first came together, to help create an event that changed all of their lives, and unleashed an energy that still resonates and ripples through Scottish life and culture, almost four decades on.

Beyond that, though, it's also a reunion between this group of people and the extraordinary piece of stage furniture around which they gather. For this is the National Library of Scotland's exhibition space, home for the next three months to the library's Curtain Up! exhibition, which celebrates the past 40 years of achievement in Scottish theatre. The group of people gathered around – Elizabeth MacLennan and Bill Paterson, John Bett and the legendary Gaelic singer and actress Dolina MacLennan, and director and producer David MacLennan, now famous as the man behind Glasgow's Play, Pie and Pint lunchtime theatre phenomenon – includes almost all of the cast members of 7:84 Scotland's legendary founding production of The Cheviot, The Stag, And The Black, Black Oil, the mould-breaking radical "ceilidh play" which first went out on tour in the spring of 1973.

And the centrepiece of the exhibition – around which they circle, giving it the odd affectionate prod to see if it's still in working order, before they move next door for a 90-minute panel discussion and debate about the 7:84 experience – is the extraordinary piece of stage design known as "the pop-up book"; the giant eight-foot by ten-foot book of pop-up sets conceived by the show's writer and director, the late John McGrath, built by sculptor and fiddler Allan Ross, and painted for the first tour of The Cheviot by the artist and future playwright John Byrne. The book's dimensions were dictated by the size of the roof-rack on the company's Transit van, where it had to survive months of rugged Highland touring through wind, rain and storm, covered by a tarpaulin. And after a hair-raising history, during which McGrath's wife and lifelong artistic partner Elizabeth MacLennan had to salvage it from more than one rubbish skip, the book has finally come to rest with its new owners in the National Library, although the building has yet to find a shelf large enough to accommodate it.

YET there's a sense that the pop-up-book's journey is not over; for what it does – as it sits in the exhibition hall, among a loosely-organised blizzard of other material from the NLS's huge theatre archive – is to raise a whole series of questions about how to integrate the story of an art-form as live and ephemeral as theatre into the history of a nation, and into its archive record. On one hand, the material gathered here is tremendously interesting and resonant to anyone who has personal memories of the shows that left behind these fragile material traces, not only in the vast 7:84 archive of John McGrath's personal papers and records, which is held here, but in other major NLS collections, including the entire archive of Traverse Theatre playscripts, programmes, photographs and company records, dating back to the 1960s.

In theatre, though, it's always the live human experience that matters, rather than the object or the text on the page; which is why the evening swoops onto a whole new level of energy when a packed audience gathers in the boardroom next door, and chairman Jim Naughtie ushers the 7:84 team onto the tiny stage, to discuss their story with Bob Tait – the man who, as editor of Scottish International magazine, staged the first reading of the play at a conference in March 1973 – and with Isobel Macphail, of the Assynt Crofters' Trust.

"I don't like looking back, really," says Elizabeth MacLennan, still glamorous in her eighth decade. "I think theatre always has to be re-inventing itself." And although she proves herself no mean archivist – producing from her bag a copy of the very pop-up book of Pinocchio that she and McGrath were reading to their sons when the idea of the pop-up set was born – she and her fellow panelists still seem strongly focused on the present and the future, as they talk about the multiple meanings of the 7:84 experience. Bob Tait urges the audience to think about the politics of the show, and about the way it negotiated the ideological gap between nationalism and socialism in Scotland, still unresolved today. Bill Paterson recalls the sheer speed with which John McGrath would recycle that morning's headlines into the evening's show, and is sure the same could be done today; and up-and-coming playwright Alan Bisset is inspired on the spot to work with David MacLennan on an update one of 7:84's later shows – possibly The Game's A Bogey, McGrath's famous tribute to Red Clydesider John Maclean.

At the back of the room, the National Library curators and staff smile; because they know that this kind of event – this living link between their collection and the life of Scottish theatre today – is essential to bring out the full meaning of the artefacts, and to save them from any suggestion of dusty nostalgia.

"We feel that it's not generally known that we hold such a big collection of really contemporary material in this area," says the NLS's exhibitions manager, Jackie Cromarty. "So we hope that this exhibition will encourage people involved in theatre to recognise the amount of vital contemporary material we have on the history and development of theatre in Scotland. And also that it will encourage other people who use the library to appreciate just how significant theatre is as a living part of our culture; and just how much it has changed and developed, over the last generation."

The Curtain Up! exhibition is at the National Library of Scotland until 3 May, with further panel discussions on the Birth Of The National Theatre of Scotland (9 February), and on the Traverse Theatre (8 March).