Tag of some sort

[LabradorBob (Missouri)]

This measures about 5/8" across.
says THE HOLLAR CO. ENGINEERS PHILA.
with an R REG mark.
any info would be helpful,
good luck to all.

 

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[Ramitt]

I would guess it is a tool check tag.

 

[seger98]

I would agree, looks like a tag to keep track of something!