Igor, Thanks for the explanation.
The site is confusing about the Cobra bearings on the 3515. The bearing kit for the motor shows 3 bearings, and the wording in the 3515 only mentions one front and one rear, "the internal bearing." The bearing kit does fit several motors, so perhaps some of the longer motors use 3 and the kit mentions several motors that it fits.
The BA bearings also fit the Cobra, so if you get the kit with correct size and number you can "upgrade" Cobras.
In addition, I have a couple of older sets of Cobra bearings. At that time, the bearing pack for the 3520 also fit 3525 and 2826 and all were 3 bearing motors. The 3515 wasn't listed on the bearing pack then, likely because it has 2 bearings??
The Cobra bearings are front NMB R-1650HH 625 ZZ (5x16x5), Class P5. That P5 is equivalent to ABEC-5 from what I can tell. As Jim noted above, BA are apparently ABEC-5. I don't see a designation on the internal bearings.