Close as I have to your M&P's SN on my list is 564xxx which shipped in 2/27. Yours may well have shipped in late 1926. M&P (Military and Police) is the model name. The plated hammer and trigger indicate that it has been refinished outside the factory. S&W did not plate hammers and triggers.

For your M&P Airweight Model 12, the book says:
D510,001 – D659,901………Late 1973 – Early 1974 (manufacture)

I don't know what "Its been said to be a collector special" means, but most Model 12s seen have 2" barrels so those like yours with 4" barrels are unusual and would have more collector interest if in high condition. They were cataloged with both barrel lengths at the same price. Sometimes these alloy-framed guns are subject to cracking of the frame where the barrel screws into the frame. Open the cylinder and see if there is a fine longitudinal crack in that area. There probably will not be one, but if there is, it should not be fired.