Accuracy Of Perforations In Older Stamps

Hello all. First time post, new member.
I have a recurring problem with some older stamps, pre 1900, and would appreciate input. Here is my most recent instance.
I have a 50c U.S. violet, Franklin stamp. This could be a Scott # 341, 421, 422 0r 440. The first three are perf 12. The 440 is perf 10.
Measuring MY stamps perforations on all 4 edges I find they are most definitely 10 3/4. I am at a loss to decide how to go forward. I have been collecting for approx. fifteen years in a vacuum so to speak with no contact wit other collectors or online groups. Until now...Geoff

It is common practice to round perforation gauges to half-perforation gaps. So, 10 3/4 may be catalogued as 10 1/2 or 11.

When a stamp is catalogued either 10 or 12 and you measure (not count!) 10 3/4, the odds are you either are not doing it correctly, misidentifying the stamps, or not reading the catalogue listing correctly.
Can you show us how you are measuring perforations?

Isn't Scott 341 a George Washington stamp?

Not knowing about US stamps: could you have missed Scott #517?

Scott rounds to the nearest 1/2 for that era. What they call 11 is actually a little lower (I think I have gotten 10.9 when I try to measure precisely and 9.8 for their 10, but it is hard to be that exact).

I agree with NSK, it is probably a 517.

https://www.theswedishtiger.com/ID341.html

Wonderful. First, yes 341 is a Washington (of course, you knew) My first mistake.
2nd. Now knowing that it is a 517. I have searched twice using different criteria and have found 517 both times. It is clear that my earlier search was sloppy. I will learn from this and move on. Thank you Gentlemen....Geoff