Firearms are generally engraved with a graver- basically tiny chisels. That piece was chemically etched, probably with ferric chloride, after being coated with a masking agent, which was then scraped away by hand, to create the pattern.
That lends credence to it being a one-off and DIY/shopmade, as anything produced in quantity would likely have been punched/stamped, not etched, and anything done professionally- as a presentation- would have been a lot more consistent and, well, professional, in the symmetry and evenness of the etching.
We could even argue that it either is, or a very close cousin to, wrought iron, as the close ups of the etching strongly suggest a 'grain' to the metal, which is characteristic of WI.
Doc.