Need info on Win. .22 EZXS specifications|Winchester .22 Rim Fire|Forum|Winchester Collector

I just picked up a near mint box of 1939 Win. EZXS .22 LR shells.  I have keyed it out on Heugel (WRA-54, “LR-3” top, “B’ bottom, “K2238R”, “H-10” h/s) and have perfect match for it being produced in 1939 (end flap code HS 39) but with one exception.  The book says brass cases, but these are, or at least appear to be, copper.  Hard to tell without scratching the surface of one.  It’s kind of strange, because these are supposed to be a high speed loading (HS 39) but brass was generally used in high velocity loadings due to it being stronger.

Anyone out there have any info on this variation?

P