I tried the same search with the (mini-14 serial numbered age )and got no results
Is to your question about the trade I think that was a good trade a preban mini is worth about 400 bucks and if it's not today it will be later because they won't make anymore prebans and as yours was made in 77 it's defantely a preban.As to putting a bull pup stock on if it's not a folder and only has a pistol grip then you should be fine as long as you don't have a bayonet lug or flash hider because that would be legal even on post ban.I'm not sure about putting the preban stuff on it even though it's a pre ban but I belive if it's ruger stuff you could get away with it.(example you buy a 77 year mod mini it has standard stock ,no flash hider,no bayonet lug, you add a folding stock made by ruger and a factory flash hider and a high cap clip .The goverment can't prove you added these things after the ban unless your gun is made after the ban.However I would'nt risk it untill I asked.
A topic titled (preban with folder added post ban?) should give you the right responce.