I have a lower unit "foot" which I think is from a mid-1950s Merc. I owned a KG7 Hurricane 10, but this foot is smaller than that. What was the 7.5(?) HP Merc called back then: KF7 ?? What do I need to measure, or were do I look for an identifier, so I'll know what I have? The prop, driveline and the long mounting stud are in place, the driveline turns easily and you can hear the impeller squeak as it turns. It measures 26-3/4" from the bottom of the skeg to the top of the driveline and the long mounting stud protrudes 11-5/8". The copper water line is broken off.

What do I have? Any help appreciated.

The 10hp models (KE7, KF7, KG7) shared the basic gearcase housing with the 7.5hp models (KE4, Mark 7, KG4), with driiveshaft and propshaft changes being the difference. The 3.6, 5, and 6hp models used a smaller gearcase, with more variations depending on whether it was the direct drive (KF3 or KF5) netural clutch (Mark 5 and Mark 6/6A).
The fact that you've got the long stud mount means it is likely a 7.5 or 10, as both used a long forward stud. A KE7, KF7 or KG7 would have a stud that extends almost as long as the driveshaft, while the KE4 and Mark 7 would only extend about halfway up.
A KE4 or KE7 will have simple slits on the side of a smooth gearcase bullet, while the Mark 7 and KG7 will have ridges cast into the bullet, leading up to the water intake slits. I've seen KF7s with either set-up.
Hope that helps more than it confuses. I am working from memory on this, so I may not be 100% accurate, but I'm pretty sure I've got the details more or less correct...