Nailed it this time 6 1/2 years on. http://www.ebay.co.u...cvip=true&rt=nc
Appears to be the standard size. Replacement Duofold nib and plastic showing some shrinkage around the top of the cap.
Imprint is purely Vacumatic and Reg TM. This last bit is how I identify English Duofolds so that supports it being English production.