you need to look under the freewheel threads to spot the difference Mike , the 550 has the black bearing side visible and the 500 has a drilled silver "dust shield"
I've got a pair of 36H going spare that are in OK condition but that weren't up to being fitted to a pair of NOS SSC rims......
Usual corrosion to QRs and surface is not so nice on front hub. Rear hub is nicer.
£45 plus postage the pair? Or £60 with a new-looking 36H 501 hub to go with them.....