A 38 Special M&P with serial number 582XX shipped July 1906 in the SWCA database, so you probably are looking at what should be called a very early Model 1905. Is there a cylinder stop screw just ahead of the trigger guard?

Added: Sorry, I just added this post as you were already posting yours. 5 screw would be appropriate for a Model 1905. Roy Jink book states the Model 1905 started with serial number 62,450, but there has been lots of debate about that date.

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Gary
SWCA 2515