That appears to be the rear axle. That is the spacer on the drive side. The collar is just a stamped steel piece that is a press fit on the spacer. They are listed as separate parts in the fiches, but hardly ever found alone.
On some of the parts sellers they have a parts look up with fiches, some have a p-lace you can put in the part number and tells all the bikes that used that part number.
All those drive side spacers were the same from 74 up. So it shouldn't be hard to find.
Leo