Not very unusual, actually, and certainly not rare. There were a lot of Swiss-made cases for the P3s and P4s (Classic and Big Time models...haven't seen a Swiss Exec yet); all the case backs have 6-digit serial number starting with "5". On the P3s, the "HR" and "MIN" markings were ink-printed, instead of being stamped into the metal, as they are on the standard Time Computer-made part (don't know why the Swiss case-maker, whose identity is still in question, elected to do it that way...cheaper, maybe). So, every time the watch was set, some of the lettering was lost to abrasion with the magnet; most are now completely blank...occasionally, some trace of the lettering is visible. I have never seen one with fully intact lettering. The rarest Swiss cases are, of course, the solid 14 and 18-karat Midas Collection models...there are many I have never seen, and it is actually possible that no examples exist of some of them, having been consumed in the fires of the Mass Melting of 1980. If I'm right, it is no longer possible to assemble a complete collection of Time Computer watches...a pity.
Side note: Case rings for the American-made P3 cases will always fit the Swiss cases, but not always the other way around, due to overlapping ranges of dimensional tolerances (same thread pitch, but differing diameters)...something to keep in mind when attempting to swap parts.