I have both of these attached to some systems. The IBM mouse is better than most and I used one for many years before switching to a trackball.
The rubberdome version of the M is quite good. It had to be to be a substitute for the clicking version. The buttons are not the same and are missing the channel for the spring, so you can't use them with a BS M. If there are keycaps, those will work with a BS version M.
The ps2 cables are available from Unicomp and changing them is relatively easy from what I've seen online.
Not a bad deal for a pair of quality components that need some tlc, although shipping really needs to be more like $10 for it to be a really good deal.
Great find, imo.