Well, finally got the right positive mindset to come back and get that puzzle over with...so my very first guess that first time I suppose would have been 3B01? Like I had every single detail figured out from the get-go apparently...except I never for the life of me would have counted right to left for the lockers, even acknowledging that I had technically gone from right to left to get to that set of lockers in the first place. In fact all of my guesses after that were just trying different places to start counting for the "3" because I had been certain on what B and 12 would mean.
Is that actually how locker rooms normally work or something? The only one I've ever used was at work and the lockers aren't labeled at all (or assigned).