ATF has long held that placing a serial number directly into polymer does not meet marking requirements since such serial number would be susceptible to being readily obliterated, altered or removed.
The final rule amends 27 CFR 478.92(a)(2) and explains that an acceptable method of identifying a polymer frame PMF is by placing a serial number on a metal plate that is permanently embedded into a polymer frame or receiver, or other method approved by the Director.

Licensees may not take into inventory PMFs they cannot mark pursuant to the requirements of 478.92(a)(2).