Or to give them their full name Bare Knuckle "Pat Pend '59 Slab Board" pickups, just rolls off the tongue that one. They're A5 single coils wound with formvar wire, and a relatively low output. With a little give on the attack and a surprisingly smooth top end they're not particularly similar to many other late '50s style single coils, if you've ever tried Fender CS 57/62s you'll know what I'm talking about: all treble, all day. These are much more usable, and don't have me reaching for the tone control even on the neck position.
While Seymour Duncan SSL-1s are probably still my all-time favourite single coils, the '59s are definitely in the running, and while they share many similarities (same magnets and wire choice) there are noticeable differences. While SSL-1 pickups are wound hotter they have an immediate attack and note bloom which gives the impression of being brighter, or at least more modern. The '59s share characteristics with vintage pickups and sound years old coming straight out of the box, the softer note attack is subtle and more of a feel thing than something that's obvious in the recording.
I've shown this '62 reissue strat before, it's the one that got some EVO gold frets put in last October, and now I've gotten around to recording what it sounds like. Neck and middle only on this one, the bridge is actually a Slow Hand (with baseplate). Starts with the neck, then both together, then finally the middle (both tracks.)
Edit: If you're considering trying some of these pickups they're extremely finicky when it comes to height, these are set to 1.5mm treble side and 2.2mm bass side with the string fretted at the last. The middle pickup is also not rwrp, I dislike reverse wound middle coils at the best of times but it really doesn't work with these.