|BMW||wet cell||61 2 12 306 626 (and others)||61 2 11 459 650|
The WestCo battery is sold by the usual BMW bike mail order places (such as Eurotech, maybe also Rocky Point, my two personal favs).
Rumour has it that WestCo merely slaps its own label on a Panasonic, but which Panasonic battery? For the SWB, a scan of old /5 emails yields the Panasonic LC-R 1220P, LC-X 1220AP, and LC-X 1220P. Substitute 1228 for 1220 for the LWB. Moreover, the LC-X 1228 has been replaced with the LC-XB 1228. What's the difference between these?
First the obscure one. The LC-XB is the same as the LC-X version, except for the location of manufacturing (Mexico, instead of China) and minor changes to the cover. Panasonic has a rather hidden website on this topic. That document also has a pretty clear presentation on the difference between the P and AP: the terminals. The AP has a threaded post for the battery terminal, while the P has a bolt and nut battery terminal. Panasonic has a very detailed description of the terminals, but you're better off with the previous link, the change notice, if you want to picture the difference. While bikes have been made to work with the AP, the P is closer to the stock configuration.
Final issue is: LC-X or LC-R? The issue appears to be significant: Panasonic says that the LC-X is appropriate for standby power applications, while the LC-R is appropriate for both standby power and main power applications. Is a motorcycle battery a standby power application? I dunno, but somehow the fact that it is parked most of the time doesn't compensate for the brutal demands of starting a cold engine. A number of those old email posts purport alternate, inexpensive sources for the sealed lead acid battery: BatteryWeb, GotBatteries, Portable Power (which appears to be the same company as GotBatteries), and at least expensive, Digi-Key. Of these, all list the LC-X version of the battery, except BatteryWeb, which lists the LC-R1220P, but according to ancient private testimonial, ships an LC-X1220P battery.
Interestingly, Panasonic lists the LC-X1220P, the LC-X1220AP, the LC-X1228P and the LC-X1228AP, but not the LC-R1220P.