Produced towards the end of the Whitney Wolverine's production (circa 1958-1962) the 37000 Series Wolverines were built on frames produced by the Bruce Manufacturing Company of North Billerica, Massachusetts, with a number of variations from the standard design. The most prominent is the rear sight, a windage and elevation adjustable rear notch, set into a pair of raised wings integral to the frame; though the individual part is more expensive than the earlier fixed sight, the elimination of the man-hours spent getting every pistol calibrated made it cheaper on the whole. Additionally, the rear of the frame was reinforced and the threading of the barrel nut adjusted, in response to complaints of the frame being damaged if the hammer is accidentally dropped during disassembly and excessive play in the barrel, respectively. Not all of the features of the 37000 Series are present, such as the overtravel adjustment screw and satin epoxy finish, suggesting that this is a prototype or demonstrator model. The left side of the frame is prominently marked "WOLVERINE", with "CALIBER .22 L.R." on the left, the Bruce "C" imprint under the right grip, serrated trigger and checkered brown grips.