I have seen it written (in an IAA or ACCA Journal) that Winchester at least made some trial lots of .310 Cadet with WRA Co headstamp. The source was unaware of any rounds in existence.
Has this been confirmed or is this still a mystery cartridge?
The ACCA Journal index lists an article on Winchesters 310 Cadet in #83 P 21. I don’t have this issue but I have just put in an order for some ACCA Journal back issues which includes #83.