There is nothing similar in the dimensions of HT and BBB chain, and as you have found out, they are not interchangeable. Simpson-Lawrence should be able to sell you a HT gypsy, though it probably won't be cheap.
Exactly. A casual measurement of one link might look close but if you measure a longer piece or look at the specs you will see that 3/8" HT and 3/8" BBB are not the same. At least in my SL windlass what matters is links/ft which is of course determined by the link dimensions. 5/16 HT and 3/8 BBB are close enough that either works perfectly but 3/8 HT definitely not.
By the way, the SL manuals are nonsense. Mine (the origional) claims that my gypsy will use 3/8 BBB or HT, 5/16 BBB or HT and 8-10 other chain sizes including metric which absolutely wrong. Spoke with John at SL Spares in UK and he confirmed the manual was marketing BS.
He is by the way, the go to guy for parts on old SL windlasses. He worked at SL for years and bought out rights and parts to all the old models when SL discontinued support.
I keep trying to tell folks that HT chain is a waste of money. The way chain works is by using the catenary curve to cushion any shock to the chain. BBB chain weighs more per foot than HT and therefore does a better job. Way too many folks think that just because HT is stronger it is better, but if you check the specks, you'll see that neither BBB or HT is strong enough to actually hold the boat, should the chain be tight between the anchor and the boat. Therefore you are better off with the BBB as it is actually the chain designed for anchoring.
Respectfully I don't completely agree with this. First I'm sure you agree that a snubber is much more effective at cushioning the shock than the catenary of the chain. Also catenary may work at lower winds speeds but when it really starts to blow the catenary on either size chain will be small enough that the effect will be effectively nil.