You have a very late 1st type Officers Model. It has the late style frame that was only used from around serial number 270000 in Dec 1906 until the end of production in 1908. The gun is most likely not original due to OM's being target revolvers and not being nickel plated. The serial number might be shallow and buffed down a bit and actually be 288XXX. That would put it in 1907 which would be correct for that style frame. I have one just like it, serial 290XXX and a 2nd type OM from 1908. The gun was originaly made for 38 Long Colt. It will chamber a 38 Special, but the internals will take a beating.