A belated happy birthday to your Lotus! A Waveset fin (or replica) would be a perfect gift for it.
Your board probably did not have a fin in it when the original owner bought it off the rack at Surfboards Galore. Usually, after you picked out the board you wanted, you would then select a fin for it, depending on what color you liked and what fin shape the salesperson recommended. With the Wavesets, they came in all the primary colors and five or six shapes. If I remember correctly, the first ones at the beginning of 1968 were opaque and made of polypropylene. A few months later they were also made from translucent polycarbonate. I believe the ones that Juan makes are made out of layered fiberglass. Anyway, the Waveset fins I've seen in a number of Lotuses, were either black (polypropylene) or yellow tinted (polycarbonate). Since Juan's fins are opaque, you might want to have him make it in black. But, it doesn't really matter, because if it was mid-1968 and you were buying the board today, you would have picked out the fin in a color that you liked best. That's what you should do today.
By the way, all the Lotuses I've seen had the high-aspect, G&S style Waveset fin in them (long and narrow).