I have a 4x10 BA, I bought it used, the speakers are black alnicos. I wasn't sure what make they were, so I called Mesa and asked them, and the guy said they were Eminences, especially made for Mesa and not commonly available.
I know that the Emi alnico speakers were one earlier option, and some sort of Celestion 10s being the other early on, then later on the Jensens were the only 10s available in a BA.
Wouldn't the Jensens have the brand name on them?
I'm actually finding the alnicos in mine quite bright when I drive them at a louder volume (even with the amp treble set low), so I have to set the tone controls on my guitar at about 7 on the neck pickup, and 5 on the bridge to dial out that shrillness. Otherwise, they sound very nice. Actually, I'm wondering if it's the Blue Angel itself that is bright and not the speakers...