these are indeed chain tensioners. Modern versions are sometimes used on BMX or "mod" trials bicycles with bmx-style dropouts (track ends if you prefer) and are commonly referred to as "snail tensioners" or "snail cams". The tensioner slips over the axle, is typically on the inside of the dropout, and can be rotated as a cam to pull the rear wheel backwards. This sort of tensioner is not typically required on bicycles unless there is a lot of torque pulling the wheel forwards (as on trials or bmx bikes) but may have been used on a motorcycle.
Here is an example in use. This one is visible in between the driveside locknut and the frame and is serrated to help hold it in place. It pushes against a dedicated fitting protruding a small way from the inside of the dropout:
http://img.photobucket.com/
threaded tensioners which pull against the back of the dropout are far more common however:
http://www.danscomp.com/
- Joel