More ID Help Please - A US Seal?

This one is obviously not a stamp, but what was it used for and when was it in use? Any help would be appreciated.

That is a Post Office Seal (Scott designation OX), used initially to prevent tampering with registered mail and, beginning in 1877, to repair damaged letters, reseal those opened for inspection, etc.

It looks like yours is likely one issued between 1946 and 1948. They used the same design but are differentiated by perf measurement and paper type. That particular design was in use on seals from 1919 to 1972.

See https://goscf.com/t/9242&SearchTerm...+office+seal

Thanks a lot tomiseksj, much appreciated.
DavidR