I looked at my catalogs and didn't see SG59...saw SG58 and SG61 and both are Mopar. Maybe Mopar? How about a picture?
If you can't track anything down, you might try contacting these guys: Speed Gems - Transmission Adapters - Proudly Produced in the U.S.A.! They acquired the adapter portion of the old Speed Gems business some time ago and may have some archival stuff that could possibly help you I-D it.
Wow, that's really cool! If you decide to let it go, shoot me a PM, always looking for that era MoPar speed parts/accessories.
It was in an online estate auction that ended this evening, and I was trying to find out what it fit before the auction ended. Never found out for sure, but I bought it anyway. Not even really sure what I ended up giving for it. Those "soft close" auctions are a trip. lol I'll find out when I pick it up in a couple days.
I know this thread is a little old. What did you find out on your Speed Gems adapter SG59. I just acquired a Speed Gems Adapter SG58 that also looks like the SG5859. Mine came of a 392 Hemi. I also got an adapter marked SG61 that looks like the SG5861. View attachment 6557554 View attachment 6557555