1cent1 Design A140 Perf.11 On 1930-S Envelopes?

Hi All!
Amongst almost a dozen envelopes addressed to a Henry W. Diedrich in Stratford, Connecticut, I have these three envelopes with 1cent1 stamps perf.11!
Now, according to Scott, the last such lot was printed in 1922 (the rare ones!!), so I wonder how can these stamps be on envelopes posted in 1930, please? For comparison, I scanned a perf.10 1cent1 on an envelope sent in 1915 ...
Doing a bit of research online I discovered that there was indeed a Henry W. Diedrich, who lived in Stratford Connecticut at the address in question. [Some of his offsprings are still at the same address, apparently.] He was, according to an old obituary, a custodian of Post Theatre.

The sizes of the three 1cent1 on the 1930 envelopes:

April 12, 1930 to Henry W. Diedrich:
1cent1 design A140 perf.11, size of design base 19.25mm x left 22.75mm x top 19.8mm x right (imperf) 22.2mm, rotary most likely.

Aug 23, 1930 with winged star stamp to Henry W. Diedrich: [The back bears the stamp "Courtesy of G.B. Camp, A.A.P.S. 45"]
1cent1 design A140 perf.11, size of design base 19.25mm x left (imperf) 22mm x top 19.1mm x right 22mm, rotary most likely.

Aug 23, 1930 with 'VIA AIR MAIL' stamp and red mono-wing plane to Henry W. Diedrich: [The back bears the stamp "Courtesy of G.B. Camp, A.A.P.S. 45"]
1cent1 design A140 perf.11, size of design base 19.25mm x left (imperf) 22.1mm x top 19.25mm x right 22mm, rotary most likely.
So, please help me make light on this one!
Thank you very much in advance!
Augustin

Many souvenir covers prepared by collectors for philatelic events such as first flights and airport dedications were stamped with older stamps. This use is very common. Survey some ebay listings for flight covers and it will become obvious.

Use of older stamps is common today as many collectors still use stamps from the 1930s on their mailings - although most likely ones with damaged gum. Here a part of an envelope sent to me a few weeks ago with Scott 703 issued in 1931 (86+ years ago!) and Scott 1207 issued in 1962.

Thank you very much, John Becker!
How can I identify accurately the stamps without taking them off their envelopes, please?

As said in your recent other thread it's better to take a known rotary and a flat stamp and take this as a template to get the size comparison. I also use mm measurements quite often but after the template test. And the perforation can of course just be checked with a perforation gauge.

Sometimes collectors have to be satisfied with narrowing the ID to 2 or 3 options (usually watermark differences) and leaving the cover intact. Please do not remove stamps from covers just to get an ID unless there is a compelling need as the value of the cover will almost always exceed the value of the removed stamp. I agree with stamperix, Flat, rotary, and offset printings can be determined from the front with a little bit of homework.

Thank you very much!
Definitely they stay on the covers :)

Since we know the two cent coils are rotary, it seems clear that the one cent stamps are flat plate; they are much smaller. Probably perf 11.

This evening (UK time, here) I will measure and compare them with the ones already identified accurately.
Over the weekend I will post the results.

Hi Everybody!
How is your end of the week?
Overnight I applied the method described by Don with a template made from a #405:
http://www.stampsmarter.com/learnin...methods.html
All three 1cent1 stamps on the envelopes are wider than the template, with at least one thickness of the borderline of the design.
Also, I studied them and compared them with other rotary, offset and flat plate stamps at different sharp angles (another tip from Don).
Then, I used some very thin plastic foil and I gently pressed it on the stamps: some of the lines were visible on the foil. So, they look very rotary press to me.
... and I checked, again and again, the perforations: 11 all over.
Now, at this point, is there anything else to check, please? [Without having the stamps taken off the envelopes - which I won't do.]
My next idea is to start comparing the perforations of my stamps with every single recorded 1cent1 rotary perf 11 stamp available online: needle in a haystack, but with a bit of automation I might be able to do it. Who knows, perhaps there is a 'sister stamp' in somebody's collection :)
Do you know anyone in UK who might be able to have a look at them, please?
Or, if anyone has a rotary press perf.11 we can compare the stamps?
Thank you very much in advance!
Best regards,
Augustin