What rear end is this? Is this a 10 bolt 8.5”? Why no bolts?

Borrowed from Rearend identification

GM removable carrier type

The removable carrier rear end was used by GM for a number of years in passenger cars before GM started using the integral carrier rear end. The Pontiac/Olds versions from the later '50s were a drag racing mainstay for many years- even in front engined Top Fuel dragsters and Altereds.
[edit] 1955-'64 carrier casting numbers

3 series (2.73:1- up): 410408N
2 series (2.56:1- down):410409N The Chevy version used various tags to designate the ratio.

[edit] Casting numbers and casting codes

• The casting number for a 1956 case should be 3707306
• The casting number for a 1957-'61 non-posi center case should be 3725899
• The casting number for a 1957-'61 posi center case is 3743833
• The casting number for a 1962-'64 posi center case is 3789812

The above are all casting numbers for passenger cars. These casting numbers are the same for Corvette rearends because the rearends were cast and assembled at just two facilities: the Detroit Gear and Axle plant and the Buffalo, NY plant. The only thing that would be different between the same year Corvette and a passenger car rearend center section would be the stamped (not cast) two letter code on the passenger side of the case. The code designates the gear ratio.

That is a 1957 Chevrolet rear end. It was the ONLY year with a drain plug. The 57 also uses different/larger axle bearings, so you need to know that for any future axle/bearing work. Gears were 3.55 with an automatic transmission, 3.70 with a 3 speed manual transmission, and 4.11 with a manual transmission with a Borg Warner overdrive. There are LOTS of different gears both from Chevrolet and the aftermarket however. The third member will swap with any 55-64 full size Chevrolet Car, 62-64 Chevy II/Nova, and 56-62 Corvette. Looking directly at the third member from the front/U-joint, if it's a Positraction case, it will have a large "P", although 57 was the ONLY year that DID'T use that identification method. Could have had a lot of things done to it over the years. 8.2 inch ring gear, and should be plenty strong, unless you really beat on it with sticky/big tires.