I Might Have Found Something Pretty Cool.....

Nifty! I haven't seen a head like this, so I can't comment affirmatively on what it is, but I can help pare away some of the guesses: it's not a marine or industrial head (same as passenger car), and there was no special factory LPG head—at least not in North America.

The weirdest part of this head isn't the nonstandard combustion chambers, it's the nonstandard combustion chambers with standard production casting numbers. Chrysler did sometimes put casting numbers and other such markings on prototype parts—the ones that were close to final form, and where production approval was anticipated. In every case I've seen or read about, proto parts got their own casting numbers; they didn't share regular-production parts' casting numbers. As shown here.

I agree the head looks like it was made this way, not modified. If it had been welded up and then ground down, we wouldnt' see the sandcast texture on the nonstandard wall of the combustion chamber.

I used to have an extensive library of documentation related to the development and evolution of the Slant-6. Nothing even vaguely like this ever appeared in it, nor is there anything like it in the list of engineering A-programs involving the Slant-6.

My best guess (still only a guess) is that this head originated outside the US or Canada, at one of the other places in the world where Slant-6 engines were made.