As it has been pointed so many times. Protocol droids and Astromech droids do look a lot a like. We have seen droids that look exactly like 3PO in all the movies. And R2 units are very similar (though I don't recall off hand any one looking exactly as R2D2). So Owen, or OB1 can't assume that that droid is the very same droid they met 20+ years before. It's like I sell my car (VW bug) to another country and 20+ years I see a VW bug with a different color drive by my street, I probably won't recognize it by just looking at it for two seconds. That aside, the locked memory works even better than most of you think. In ANH, Leia uses a "security card" or something like that on R2 before she records the message. Could this card that she inserts in R2 be the method to unlock its memory? It could work!. Also, 3PO says, I don't know which planet we are in (to Luke in ANH), and Luke doesn't say, we're in Tatooine. Of course the fact that 3PO doesn't remember the homestead or the Lars is a long stretch, but maybe he did got his memory locked in EP3. And yes, OB1 has yet to meet 3PO, unless Anakin has explained to him about how he built a droid when he was a boy.