I have this incoming. (Seller's pictures) The thing it seems to most resemble is the spring retainer plate fitted for parachute jumps, but the bits sticking out are rather odd and it also looks thicker than normal. The seller seems to be selling off relic gun parts dug out of a dump pit.
All suggestions gratefully accepted.
Nature and nature's laws lay hid in night;
God said "Let Newton be!" and all was light.
The appearance may have changed a little during an extended "dirt-nap".
Un-assembled parts sold off as scrap post-war? With the Patchett / Sterling coming on-line, all manner of odd bits were probably sent to an ignominious fate; dumped at sea, melted down, etc.