i recently bought a 39a with a case hardened receiver with serial # b12** any info on age or value?

I'm not sure this is correct, but according to http://armscollectors.com/sn/marlinlookup.php , if you input the first two numbers (not the letter), the year for 12 is 1988.

Hopefully someone with more Marlin knowledge than I can weigh in here.

By the way, I edited the serial number to conceal the last two digits. Probably not too safe to disclose the entire serial number on the internet.

Alyeska338's formula for 39's is correct for models made after 1973.

However, a case colored receiver and the B prefix would indicate this rifle was made 1940-1941. This is referred to as a "Prewar Variation".
Value from the "2005 Standard Catalog of Firearms" by Ned Schwing follows;

Excellent- $1200
Very Good- 800
Good 450
Fair 250
Poor 150

HTH

Dave