My solution: modify a cheaper version of the clippard to look like an R331.
You'll need access to a mill and a lathe to do this.
1) Aquire a cheap clippard valve with a plain top (R421 for example). These can be had for as little as $10 on ebay.
The body of all clippards look the same, as does the front label, the only difference is the model number and diagram on the back of the label--doesn't bother me
2) unscrew the aluminum cap off your valve, and chuck it into the lathe (don't worry about marring it, that's just some extra weathering). You'll need to machine off the "clippard" engraving, a real R331 doesn't have an engraving. If you leave the engraving there it will intersect the hole you cut for the adjustment knob later. If the engraving is too deep you may need to touch up the bevel on the edge of the cap. NOTE: you may be tempted to sand the engraving off, but I suggest against that, it should be machine faced to look correct.
3) Machine a 19mm hole in the aluminum cap
4) Using a 3/4" bar of aluminum, machine the adjustment knob
- Total length of the adjustment knob should be 23.5mm, this will make it the right height when set into the valve body
- Exposed knob is 14mm high and 18mm diameter, machine down the lower, unexposed part of the knob to 12mm so it will fit in the valve body. NOTE: my measurements aren't exact, but close, and convenient as you can use a common 3/4" endmill to drill out the hole in the cap.
- You can either machine a 4x1mm nipple into the end of the knob, or (for ultimate realism) drill a 3mm hole and push in a knurled steel pin (pictured below).
- I find that a 1.5 mm pitch knurl wheel is pretty close to the original. Knurl is about 9.5mm high.
5) Epoxy the knob into valve body, being careful to center it in the hole.
Done! At first the new aluminum part will be shinier than the rest of the clippard, but it will oxidize with time and match perfectly in the future.
Quick tip: you can pull all of the "guts" out of the valve and replace them with a 20mm plastic puck to glue the adjustment knob on... this will make the valve extremely lightweight without changing the look at all.