
Nope, not rare at all...perfectly standard P3 caseback. I've looked through my parts, and found three variations for both the stainless and goldfilled backs.
In goldfilled, I find:
Time Computer, Inc. 1973 *14K Goldfilled
Time Computer, Inc. USA *L80 Micron
Time Computer, Inc. USA *14K Goldfilled
In stainless steel, I find:
Time Computer, Inc. 1973 *Stainless Steel
Time Computer, Inc. USA *Stainless Steel
Time Computer, Inc. 1973 Stainless Steel (no star before Stainless Steel; these are the Swiss-made casebacks that always have a serial number beginning with "5" and the "HR" and "MIN" markings are painted on, rather than stamped into, the metal, so are nearly always missing, leaving the magnet recesses smooth and blank). The lettering is larger, in a somewhat blocky type font, and shallow...almost looks etched, rather than stamped. The serial number digits are smaller, and very lightly done, almost like shallow engraving.
The little circle with a "c" inside that always precedes the word Time is simply a copyright mark.
Serial numbers below 110000 seem to be quite uncommon, and numbers in the 700000 block I would actually call rare.
There are also a few variations in the P2 casebacks. A couple true rarities among those, too.