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This is a odd rat. Never seen one. model F-710-S. Date code 1951. It looks like the end of an extension, but the ext. is unfinished, has a round hole. If anyone has some info on this, please post. A picture of this shows up in the 54 industrial cat.
It has to be factory, the model number is stamped where the ext model number would be, also it has a round hole, which would be the way and ext would be before finishing.