First point of call should be The Mitchell Library. They should have records of yachts - probably through files such as the club - I cant remember the name - but was based at Hunter's Quay.
The Clyde Cruising Club might be worth a try or McLarens bookshop in Helensburgh did have a good collection of old Clyde Yachting books.
Or try this great site - you may have to be a member to view some pages.
You might find a starting place for some by using the CYCA register of local sail numbers. Especially in the 'C' series, lower usually means older (but certainly not invariably since these can be retained by owners). Along with class of boat, handicaps and last know owners' names, they're on: http://homepages.rya-online.net/cyca/BOATS.HTM
And since they'll invariably need specialist yard attention from time to time, Silvers Marine and John Hill (Renfrew Boatyard) are a couple of the more obvious places they might end up.
Thanks for pointing out that site - did not know of that and had fun looking at all 9 oyster 26s that were on the list of which we have one - not bad out of a national population of 53
Thanks to all for replies there are some ideas that I will follow up.Any more keep them coming. Our sailing club is 100 years old 2006 and we were looking at the possibility of finding some boats that were around at the time.