X-Mailer header mapping to actual version of mail.app used to send message
Simple answer - no, there is no easy way to tell from just the number.

However, I did a Google Image search for screenshots of the "About Mail" window, and found the following correlations between Mail version numbers.


Mail 5.2 = 1257
Mail 5.0 = 1244
Mail 4.3 = 1081
Mail 4.0 = 1075
Mail 3.6 = 936
Mail 3.5 = 930
Mail 3.4 = 928
Mail 2.1 = 752


If you're wanting to match to versions of the OS:


Lion: 12xx
Snow Leopard: 10xx
Leopard: 9xx
Tiger: 7xx


"About Mail" images found on these (and other) pages:


http://www.ecu.edu/cs-itcs/mac/Apple-Mail-Settings.cfm
http://gseis.ucla.edu/administrative-support-units/educational-technology-unit/etu-documentation/email-apple-mail