the rule of thumb is that you size down half size. in my case, i do not
Alden for BB PTB is indeed on the M-58 last. The M-58 last is very similar to Barrie, but perhaps a TAD more snug. Hardly noticeable in my opinion, however. I wear the same size in Barrie and M-58. I have 8 pairs on Barrie and two on M-58 and I'm 9.5D in all 10 pairs.
While we're on the topic of Alden BB PTB, I should also mention the following differences: Alden for BB PTB has no storm welt, thus has, IMO, a sleeker profile. It also has matching eyelet covers, as opposed to the blind eyelets on the factory PTB. These are a matter of preference, but all told, I'm a bigger fan of the Alden for BB. Some don't like the eyelet covers, but to me, once you wear them you will decide whether you like them better or not. I didn't like the eyelet covers at first glance, but now I love them.
Come to think of it, I also didn't like coffee, scotch, cigars or dark chocolate the first time I tried them, either.
Others may disagree and that's cool, too.