Number Out Duplex Cancels

Can anyone explain the number at the top, between the dial and the killer? Can anyone show other examples of a number in that position?

I have seen this style of barrel duplex from Newton Center, Newtonville and West Newton, Mass. with numbers 2, 3, 4, 5, 8 and 9. I haven't seen 1, 6 or 7.

I've seen two barred ellipse duplexes from Greenfield, Mass. with an 11, both from 1953. The other one is on a regular cover, so I don't think it's being on a facing slip means much.

I haven't seen any other number out duplexes.

I think that the 'two bar' cancel on the facing slip (Greenfield) is simply part of another strike from a facing slip above this one and/or mis-strike.
Don