I'd say your 'crown w' was a good call. I think the dutch Wilhelmina proof was a line worth looking into. The ones I found have a crown over a script W, but your could be an early version.
Second maybe it's not a proof mark but an acceptance or service mark.
added additional looking found this lead cut and pasted (sorry the photo was photobucketed)
The mark you are refering to is in fact now known as the 'Woolwich Stamp'.The photo attatched is from a document - "List of Stamps Used by the Department of Inspection of Munitions". My copy is dated April 1918, however I have seen a re-issued list dated 1939. It would seem that when British Inspectors went to America in 1939 to examine munitions that were being manufactured there, they took their stamps with them. Up until now this marking has been somthing of a mystery, but the list confirms that it is a British Inspection stamp. So far it has only mainly been noted on small arms were a contract for supply was entered into, such as Colt 38/200 revolvers, Smith & Wesson 9mm Light Rifles and Savage made 1928 Thompsons: although not the very first order I susspect. Interestingley, I have seen the stamping on two Colt Woodsman pistols in the Pattern Room, were no contact for supply is known but an off the shelf purchase was made. The full story of the Woolwich Stamp has yet to be uncoverd."
Edited by - charliemeyer007 on 11/02/2017 10:36:38 PM
After doing some research. this is definitely the woolwich mark. the mark has only been found on firearms made under contract for the british purchasing commission. The biggest problem seems to be that commandos are not on the list of firearms that would have had this mark. but are found on colt official police revolvers which is basically what a commando is. I look forward to the colt letter. hope it has some useful info.