1947-1958. There is sometimes a lower case letter in parentheses after the 151M - Like 151M(b)- can narrow it down some.
Go HERE: http://home.epix.net/~damguy/models.htm
Then scroll down to the 151M entry. And look at some of the photos on the way- you'll see why we collect those.
As far as the wood- avoid sanding except in extreme (was attacked by woodpeckers) case. 0000 steel wool and an oil finish for most. If the old finish is cracked, flaking, get some Formby's Refinisher (not stripper- be careful, flammable as hell) follow directions, then finish with satin Tung oil.
Before that HORRIBLE boating accident I had 51Ms, 151Ks, 142s, 152s, 146B(b)s, 44 USs and a few others from the 300 series.