Radiator Cap ID

[SgtSuperbird]

Hello all. I try to do my own research before posting, but this time, I'm unsuccessful. What exactly do I have here? The 16 is facing outward and the lettering seem to be upside down to what I'm finding as reference - and the button is not brass colored. SM CO and a patent number (very faint) are stamped on the face. Any help?

Thanks.

 

[SgtSuperbird]

Yes, I understand if I rotate the cap, the lettering is not upside-down, but the 16 is then on the opposite side. I see most on the left side. (Unless I'm confused.)