80s pop out fins non glass on loose

Hi wise elders of the ocean spray does anyone have knowledge on how the 80s fins were inserted into there fin boxes as I have a loose one rather than strip and put new boxes in trying to do a proper restore , I adopted a 1980s chapter surfboards Devon twin fin  basically a Mr twin UK clone I have an advert of the same board from 1980s in restorable condition my arms out of action with a damaged nerve from last season so I thought hey why not get something for inspiration will post pics up soon 

Here’s a pic of original advert it’s the 6ft 2 things a micro log red board second from left, a bit of history I guess 

Others here would probably have much more info about this… but it sounds like it might be the old Star Systems boxes that were common with G&S boards at that time. Would probably need a pic of the fin / box to know for sure