Its a round slightly domed disc that blocks a hole that's necessary in the casting process. I deliberately called it a core plug in the hope you'd know what that meant meant. Dunno how big a "quarter" is but the plug in my '84 6hp is about 2cm or so diameter. In the diagram on the brp site for my engine it is is clearly visible but I cannot see it in the diagrams for a 79 4hp or the earlier '75 version either. Look for a disc about that sort of size slightly recessed in a hole. Won't be in the head I don't think. If there is one it'll be in the cylinder block.tell me what the "welch" plug is.
but rather more than ?6 if its only the nut you wanted!?25 not bad for recoil assy, hood, flywheel nut and some gasket paper