Impregnated waveguide?

[Zap_50]

Hello to the group.
Removing the section of WR75 transition waveguide that attaches from the BUC to the 6" conical feed horn, I found both
the round C120 flange on the feed horn and the WR75 square inside surface flange (not visible on the outside) are stamped with red ink 'impregnated'

My searches have not found an answer to "impregnated with what?" Hopefully someone here can enlighten me on something I have
never seen......

Thanks in advance,
Pete

 

[Titanium]

EMI / RFI gaskets are conductive and die cut from silver impregnated material.