Rostock RAMPS 1.4 endstop adapter.
This makes the endstops which come with the RAMPS 1.4 (from reprapdiscount) work with the original Rostock idler.
Print 3 off, mount the switches with M3 bolts, and mount the adapter to the idler on the outside hole. If there is any confusion, I have added pictures of the finnished product.
Rostock 3D Printer
The Ultimate Rostock Printer Thing Collection
Rostock / Delta