Unfortunately the Calcutta 50 spool is a compeltely different design. That reel did not have the Super Free feature. The spools are very different between SF and non SF model reels. The overall dimensions are different as well. I wish we had something to help you out. There might be some aftermarket spools sold in Japan, but they will be very expensive given the current exchange rates.
Sorry to hear about the loss. They can't make you a new thumb? With all the advancements in modern medicine these day you figure the Prosthetics industry would be capable of anything. Forgive my ignorance on the subject. I assume too much at times.