Hi gjamison
No, it isn't "a left over frame from during the war." There apparently were some of those, but most, if not all, of them were shipped in 1946. By the time your revolver shipped, more than 200,000 M&P units had been assembled since the war ended.
We have recorded several with the C prefix serial numbers that still had the one line address. The highest serial number located so far is C8583, which shipped in July, 1948. The first C prefix unit was assembled on March 22, 1948, before the change to the four line address. Clearly a fair number went into the vault before the additional lines were added to the right side of the frame. I'm sure more of them will show up in the future. We can estimate that there were somewhere around 9,000 of them produced.
One question about yours: Are there patent dates on the top of the barrel? Some of the very early C prefix M&Ps were assembled with barrels that had patent dates. The practice was discontinued on barrels forged sometime in 1948. I have never found an S prefix M&P without patent dates, but some C prefix units did have them. All of those carry relatively low serial numbers.