You're almost spot on, Sam! "M" (large cap) stands for macro, while "m" (small cap) is for micro.

The becquerel is the derived unit of radioactivity in the International System of Units (SI), symbolized Bq and equal to one disintegration or nuclear transformation per second. Reduced to base SI units, 1 Bq = 1 s-1. The becquerel is named after Antoine Henri Becquerel, the French physicist credited with the discovery of radioactivity. In the 1970s, the becquerel replaced the curie as the standard unit of radioactivity.
The becquerel is a small unit. In practical situations, radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq), where:

1 kBq = 1000 Bq = 103 Bq
1 MBq = 1000 kBq = 1,000,000 Bq = 106 Bq

Curie: obsolete unit of radioactivity, equal to 3.7x1010 disintegrations per second. Replaced by the Becquerel

37 billion Bq = 1 curie