The S prefix M&P revolvers reached S999999 in March, 1948 (that one shipped in June, 1948). On March 22, 1948, the first C prefix revolver (C1) was assembled. At that time, the one line address was still in use. The first few thousand C prefix guns still had the one line address. I've found them well into the C6000 range, with yours being the highest number I have in my database.
I've not been able to find the number of the management order, nor its date, that authorized the four line address, but it was implemented sometime after March, 1948. I have never located an S prefix M&P with anything but a one line address (out of approximately 15,000 units in my database).
Other issues on the early C prefix units include the disappearance of the patent dates on the barrel. Some early C prefix guns have the dates, but most do not.
The high speed hammer was introduced late in the S prefix run, so one expects the C guns to have it instead of the long action. However, there are a few C prefix M&Ps with the long action, but IIRC the only examples I've seen were chambered for the .32 Long.
The highest number .38 I've found with the long action is S996765 (shipped in June, 1948). The lowest number with the High Speed hammer is S990184, which shipped to Brazil on April 7, 1948. But aside from that one, the lowest numbered unit in my database with the HS hammer is S990806, which shipped in March, 1948. Again, there are .32 Long examples with higher numbers and the long action, and those units shipped in April, 1948.
Much more than you asked for, but maybe you find it to be of interest.
