JLC was using several different buckles during the 60's, 70's and 80's.
Here are some of mine - the first is pre 1960 unsigned.
Original starps are also a nice detail!
What's common with all I have found so far is that they are very thin.
Not thick, built up like today's straps.
BTW, did you know how to check what size you are?
Me, I am size "E"!
Best
Blomman