That transmission is EXTREMELY rare. If it's an original one, it's a G76/27-A5. There is a reference made about it in the BBB as an option for the 3.5 Coupe but apparently none were equipped at the factory. A very few official cars (German gvt, Embassy cars) apparently came equipped with it, as well as a few r107 3.5. So finding an original one of these transmissions is next to impossible. It can be reproduced though, by combining a G76/27-A (3.5 four speed) with a 5 speed as found on many 6 cylinder cars of the era. I did it for my Coupe. The trans becomes absolutely identical as one of two versions. That's right, now that I think of it, there were two sets of ratios available for these transmissions.
Is that an original 3.5 Coupe five speed? If so, was it on this 3.5 Coupe from factory? If so this would be the absolute first I hear of. I would then be interested in seeing a copy of the data card if available, to see how it is reported on it. Or is it a reproduction like I did? Very nice car in any case!