This is a short barrel P.38.  It is a byf43, # 6037g. Warren Buxton has examined this gun twice and said it was the only short barrel P.38 that he ever examined that he feels may be original to the WWII period.  The proof on the barrel block probably indicates a rework shop of some sort.  The pistol is listed in Buxton's Volume II but the serial number was mistakenly recorded as 5037g.