I'm confused here. Why the need for 'pre-ban'?
After all, the high-cap legislation has lapsed. Additionally, pre-ban mags were not marked with manufacturing dates. A magazine for a Browning Hi-Power made in 1935 functions the same (and probably looks the same) as one made yesterday.
I have a few Sarah Brady mags for my USP. Mine work fine, but I'm always looking for additional hi-caps. The date of manufacture never struck me as an important factor.