Ron - there are at least two other headstamps with the word “PROOF” spelled out on the headstamp. One is a .303 British found in the original loading as well as on a crimped blank. They were made in Canada and have CWB cases, and the headstamp “DAC 1944 PROOF” with the “C” in the “DAC” having the broad arrow within the letter.
There is also a .380 Auto, and some other pistol calibers that I do not recall and just cannot search out in my notes right now, with the headstamp " + 380 PROOF" Oddly, the actual mark where I show a plus sign is the NATO mark. The " + " is enclosed in a circle. These were reportedly made in Spain, and have not other identification to show they are a proof load and no date or factory designator.
I think the Spanish one also occurs on .38 Special, and perhaps one other caliber. Just not up to going downstairs and researching it. Perhaps someone else knows. I think they are fairly scarce. As far as I recall, I have it only in .380.