According to what I have read there are only 910 of the 5" variation
That isn't accurate.
One of the shipments of the .32 M&P to Sloane's Sporting Goods in NY on April 21, 1948, contained 910 units,
most of which had 5" barrels. I actually own one of those and a picture appears below.
However, that is not the only shipment of the .32 M&P, and other shipments also included units with the 5" barrel. We don't actually know how many were produced in total, nor how many of them had the 5" barrel.
In addition, yours has the High Speed hammer. The April 1948 units all had the long action, so yours is not one of those. Notice the long action on the one in my picture.
There were some .32 M&P units with 2" barrels that were purchased by the Chinese in Shanghai that had the High Speed Hammer. One known example that did not go to China is serial number S998587. It shipped to Springfield, IL, on August 12, 1949.
I also have in my database a .32 M&P with an unknown barrel length with serial number C54973. So evidently there were some .32 M&Ps made after March 1948, when the first C prefix guns were produced.