I don't think it's a standard size. If it were I would stock them as this part is often missing.
OK scrounged around a well established LDS and got a replacement #27 washer off a junker USD reg.
I can see now that this washer acts as a replaceable lever pivot seat, harder than the brass housing... which potentially could wear a groove over time.
Bryan is right that they are a non-standard size... SAE/ANSI list a 5-40 washer as being:
OD: 0.406" (10.3 mm)
ID: 0.141" (3.58 mm)
http://www.rfcafe.com/references/genera ... rs-sae.htm
Just in case others need them during a restoration, washer #27 dimensions are:
OD: 0.3020" (7.66 mm)
ID: 0.1385" (3.50 mm)
Thickness: 0.0205" (0.54mm)
All back together now, in setting the IP at 130 psi for 210 BAR (3045 psi) the pressure flicks back smartly on purge and holds rock steady.
However, when the tank is at 100 BAR (1450 psi) the IP drops to 115 psi... is this a normal characteristic of the unbalanced first stage?
I left the IP set up for the higher tank pressure, and the whole reg breathes really well!
My first overhaul was really satisfying... I can see more reg rescues in the future