Not writing off vintage pens at all - I write with my 1930s Sheaffer Lifetime OS Balance pens most of the time. The issue with buying vintage stuff is just they there’s much variation - not so much in quality - at least not with the big name stuff - but in writing characteristics. At one point I had two 1935 Sheaffer OS Balances with fine nibs in the same color (Marine Green) and the nibs were totally different - both in the size of the iridium and in the rigidity. That’s not a bad thing if you can try before buying or are fine with just rolling the dice - and actually, since preferences vary so much, this can be a good thing - but it can be a bummer if you want to know what to expect if you’re buying via eBay.
I have written with some old PFMs - they’re very finely made pens, I just don’t dig the aesthetics. I also don’t dig the aesthetics on the “51”, but I own a couple because I came across some examples that write so, so nicely. All I was saying is that the consistency of vintage American isn’t on par with the contemporary Japanese stuff. That’s all.