Check the gun being offered, the 6M was available for a short while with Walnut target grips, they just aren't very common. Having said that, my model 6M with plastic grips is a joy to shoot and I don't think the wooden grip offers any real advantage over the plastic except the palm shelf.
The 6 comes from "model 6" and the M is reputed to mean "Match". The model 6 pistol was designed for 10m target shooting from the outset, first as the plain old model 6 with the grip moulded in with the frame. The 6G came along with improvements to the rear-sight and separate screw-on grip halves. Finally the 6M came with an improved foresight, barrel shroud hand protector for cocking and several different grip styles. The latest of the 6Ms had wooden target grips with palm shelf similar to that fitted to the succeeding model 10.
All models use the same Giss opposed piston recoil damping system and internal parts are interchangeable. The barrel changed on the 6M to accommodate the new micro-adjustable foresight. The barrel weight sometimes found screwed to the underside of the cocking aid comes from the model 10 and indicates a replacement shroud has been fitted. (or the original modified).