This part with the cutout difference may not be so oddball.
I looked at the parts for SNJ #22 and it matches your part-
Next to the cutout is some stamped info-
The E. I. seems to relate to a modification made, perhaps Engineering Incorporated or something.
There is another place a hole in the SS firewall was covered by a spot welded piece of metal. It also has an E.I. and a number stamped in it-
When they were done I don't know. A/C was sent to Corpus Christy for O/H in 44. There is a logbook entry back to 42 or 43 relating to removing the Mixture Indicator Instrument system. My guess would be CC in 44 although the 43 stamped might be when it was done or when the mod was engineered..