That is interesting, Mike. Do you have any pictures of a commercial M&P with ONLY the Service Cartridge marking? If so, does it accept .38 Special or only .38 LC? (I am familiar with the dual marking.) The first 50 or so Model 1899's had no caliber marking at all.
As far as I can determine, the Navy ordered 1000 of the "Model 1899" in .38 LC (only) on June 25, 1900. The guns were S&W serial numbered 5001-6000. Navy serial numbers 1-1000 were put on the butt instead of the S&W serial numbers. The Army ordered 1000 guns on Feb. 18, 1901; they were delivered on April 20, 1901. Those were also in .38 LC (only); .38 Special won't fit. S&W serial numbers were 13,001-14,000. The Army did not use its own serial numbers.
So those early guns and later guns made for the military were made specifically for the .38 LC because that was the service cartridge. I could find nothing indicating that any guns were made for the commercial market that .38 Special won't fit.
Obviously, those military guns were not the first. S&W must have made 5000 guns before filling the Navy order, and 7000 more before beginning on the Army order. Clearly, S&W was proud of the new gun and hopeful of a large military contract so much so that they dropped civilian production when they got a military order. Still, they had made 25,000 by Aug. 8, 1902 (including the 2000 for the services) and had made the first upgrades.
Whether they made any M&P's for the commercial market in .38 LC ONLY appears to be in question. Certainly they could have, on special order at least, but did they? That is not clear, and perhaps you can clear up the question.
Jim