Dog markings are quite desirable from any era.
I agree that the rubber stamp was likely available generally to first and second class offices (those more likely to have city delivery), as is does not appear in the list of rubber stamps available to for order in "List of Postal Supplies Furnished to Post Offices of the Third Class" (July 1, 1952 edition), or a similar Fourth Class list dated 1948.
Here is the dog text in the 1913 Postal Laws and Regulations volume:

And from the 1932 PL&R:

And from the 1948 PL&R:

Public service card (front and back) c1990 concerning dogs:


And lastly a slogan machine cancel cropped from the full cover:
