The Nikon F/FE/FM/FA SLR eyepiece will also fit and is almost identical, and cheaper. eBay is a good source.
This eyepiece can also be replaced by the corresponding screw-in Bessa or Nikon eyesight correction eyepieces (available in various strengths from Voigtländer dealers and eBay).
The Leica magnifier will not fit the Epson R-D1 viewfinder. However, a ×1.3 viewfinder magnifier is available from Japan Exposures, with inbuilt diopter correction (Figure 1). When wearing glasses, only the 50 mm framelines can be seen; without glasses, the 35 mm framelines are also visible, just, but not the 28 mm framelines. The shutter speed display can be read with and without glasses, but you have to look downwards.
The viewfinder magnifier improves focusing accuracy, increasing the maximum aperture by about a stop (e.g. f/2.4 for a 90 mm lens instead of f/3.4 without).
Owing to patent restrictions, the magnifier cannot be ordered from Germany or the US (soft of—not for use on a rangefinder camera at least).
There’s another magnifying eyepiece that fits the R-D1: The Nikon DG-2 (with the adapter for the Nikon F-series cameras). This eyepiece has a ×2 magnification, and flips up out of the way when not in use.