I believe that only the first couple of thousand of .38 M&P's in 1899 were chambered for the .38 Long Colt. The .38 S&W Special came out at that time and everything since has been chambered in .38 Special. I have no idea what one of the originals would be worth to a collector. Post some pics of it on the S&W Collectors Forum if you want a good estimate.
http://smith-wessonforum.com/