Page "Epoch (astronomy)" Paragraph 38

To distinguish between calendar years and Besselian years, it became customary to add ". 0 " to the Besselian years.

Since the switch to Julian years in the mid-1980s, it has become customary to prefix " B " to Besselian years.

So, " 1950 " is the calendar year 1950, and " 1950. 0 " = " B1950. 0 " is the beginning of Besselian year 1950.