The stamp has the look of a typical U.S. merchant or tradesman simple counterstamp from around the 1840's-50's, of the style seen on some Spanish Colonial 2 Reales, as well as large cents, Seated quarters, etc. The stamp seems to be "prepared" (to be that word... not just individual punched letters which would not be perfectly even as this one is). As Ambler, PA is just north of Philly, a major source of counterstamped merchant pieces, that might be a good clue as to its origin.
I don't have the Brunk or Rulau books, which are the main reference sources for these, but no "AMBLER" is shown on the exonumia.com master list index, and nothing shows up in a quick recent auction archive search. Random "unknown" stamps are, however, encountered somewhat regularly.
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