I'll get the party started. It's a 'hydraulic unit' for a floor jack. It looks like it's for a 1.5 or 1.25 ton Hein-Werner. The large cylinder is the oil reservoir. Hein-Werner used to use a round spanner nut(like the one in your post) , to retain the reservoir to the body block, but later changed the nut to a hex head version. The link shows the top and middle units with the spanner nut and the bottom one has the hex nut. These are rebuilt units in the link, so I don't know if they're a mix/match of parts.

I'll get the party started. It's a 'hydraulic unit' for a floor jack. It looks like it's for a 1.5 or 1.25 ton Hein-Werner. The large cylinder is the oil reservoir. Hein-Werner used to use a round spanner nut(like the one in your post) , to retain the reservoir to the body block, but later changed the nut to a hex head version. The link shows the top and middle units with the spanner nut and the bottom one has the hex nut. These are rebuilt units in the link, so I don't know if they're a mix/match of parts.

References:

http://www.theserviceco.net/repairs