To play catch-up with select examples of a few previous numbers...
44 from Chicago, Illinois which used a series of handstamps with numerals in the killers similar to the 1900s-era Doane handstamps, but these are much later:

45 in parcel post style roller cancel from Dallas Texas:

47 in killer of a steel duplex handstamp:

49 in parcel post style roller cancel from Cincinnati, Ohio:

50 in steel duplex from Chelsea, Massachusetts. One of the numbered Boston Zones starting in 1920:

And advancing the series to 51:
51 in steel duplex from Revere, Massachusetts. Another of the numbered Boston Zones starting in 1920:

51 in killer bars, no doubt another of the Chicago, Illinois handstamps like the 44 earlier in this post:

Doremus machine cancel from 51st Street Station in Chicago:
