Any era, so long as it's complete. The problem with the earlier Fibes drums (I looked for an original one for years before I bought a new one) is that they usually lose the one thing that makes them so good - the strainer. And the model that people want is the fiberglass shell, as has been pointed out. It's a unique sounding snare - very very little coloration in tone, very clean and articulate, truly able to play from the edges, from pianissimo to FFF without hesitation - thunderous if you want it to be. A very neutral, crisp, sound, which just might be why Buddy liked it. I'd stack a mint-quality Fibes COF snare up against virtually any other production snare drum, I think they are that good. They came from the Austin shop with Ambassador heads top and bottom, by the way.
I own one, an Austin-era drum (with a very low serial number, purchased from Harry Cangany). I compared my Fibes COF drum to a earlier model right after I bought mine years ago and was surprised - I found the quality of the Austin-era drum was better in virtually all respects. The tricky part is the adjustment of the butt, which is probably why so many get retrofitted with after-market strainers. It takes just a few minutes and a tool or two to get it right (And then you never need to touch it); in fact, Fibes (Austin) changed this part of the strainer twice. I prefer the open channel, but the ones built after mine had three holes on each side of the 'bridge' that connects to the lugs. I never thought it was an improvement other than to make it "idiot-proof". If memory serves, the last ones had the open channel again - but don't hold me to that.
They aren't easy to find. Call everywhere and make sure it's complete. They are bullet-proof, assuming they are original and not beat up or abused (and that should include the chromed spring steel wrap!). And beware that prices, even for later ones, are rising: I had a guy offer me $800 for mine not long ago. I thought he was nuts.
I would not hesitate, assuming you actually want to play it rather than collect it (and even that might not make any difference) in purchasing a late-model Austin-era Fibes snare. I'm putting mine away - that is, assuming I can find a snare I like as well. There is no question this is my favorite.