Thijs - I took the defroster outlet fitting off my old heater unit from my '51-3100. It does not look exactly like the one in your photo. Take a look at the attached photos and see if this is what you are looking for. It has no part number on it but the tag on the heater unit says: " Mfg. by Harrison; Lockport, NY; H-02-47". Hope this helps!
