Also, one good test to see if B 1 is indeed the almanac is to see whether the data changes significantly over time. Almanac data should be fairly static (it is valid for 180 days).

Never mind my previous assertion that B 1 being mostly 0 means that it is not the almanac. The almanac's size is fixed, and perhaps it is designed to hold information for a lot more satellites than are currently launched. So zeroes could actually make sense.

Perhaps B 1 is the almanac and B2 is the ephemeris. The size of B2 is roughly in the right range (600 bits x 12 satellites = 900 bytes).

B 3 - B 6 could be the location, time, etc