[Amps] How do I determine Class

[Jim Isbell]

On pages 202-205 of the 1956 "Radio Amateurs Handbook"  (ARRL) there is 
an amplifier described called "Parallel Tetrodes in a High Power 
Amplifier"  How do I, from that description and schematics etc., 
determine in what class the amplifier is operating?  And what is a 
National MB-40-L?  I assume its no longer available but also assume that 
it can be reproduced.  This looks like just what I need for a linear to 
go with my Viking Ranger to add some boost to the signal when needed. 
AND still be contemporary period.

Jim W5JAI