Thanks Jeff, Dave, and Bob for the comments. I found it this weekend while digging through a parts box looking for something else. I'm pretty sure I remember it came with the extra parts when I bought 6S1757 in 1987, an original extension was there too.
The car was converted to a B/P race car in 1973 and the original sender and extension were removed in favor of a mechanical SW gauge. Something I can never prove of course but since the OE sender was removed so early I guess it's not too far fetched that this is the original, the owner at the time was very good about keeping the original parts. If the "A6" stamped on the base is a date code it fits the car well...
Anyone ever tried having these replated in gold zinc dichromate?
Thanks,
Dave