Sorry to dig up and old thread - but I am trying to solve a problem I have on a 1989 Peugeot that has adjustable Simplex Dropouts -

"this version had the dropout adjuster screw that threaded thru a "trapped" black plastic plug (not shown in this pic)" quoting unworthy.

Can you still get the plastic plugs, nuts anywhere?

Or is there a substitute. Mine on the drive side has split, making aligment of the back wheel when replacing it that much harder.