My theory on the gold inlaid safety is that it was used on an early series of guns, then abandoned because of cost. Browning clearly had problems with sales, and made a series of changes about 1935 to save costs. These included the really ugly treatment of the foreend retaining screw on the lighning series in 1936, and the guns with no or almost no engraving. Perhaps the gold was dropped earlier for this reason. I have only seen it on early guns. Another feature you did not mention is the checkering, particularly on the foreend of the standard weight guns. On the early guns it was more extensive and wrapped completely around the foreend. It also seems to be finer than checkering on later guns.
