A-20 = Boston
A-16 = Hudson
http://www.diggerhistory.info/pages-air-support/ww2-allied/lockheed-hudson.htm
Lockheed Hudson US Air Corps assigned designation was A-28 however the US Air Corps never used the aircraft
A-16 was part of RAAF serial numbers http://www.wpafb.af.mil/museum/research/attack/a4/a4-37.htm
Each service used it own official designation system.
The USAF use a letter and number A20, P51 etc
While the RAF used a official name designation system Boston, Mustang, etc. ...
The US in the early part of the war started to use a official common "nickname" for there aircraft but the letter/number was the official designation