I picked up about 70 mail order lessons, produced by National Schools, Los Angeles, CA, subject of Practical Diesel Training.  The lessons are about 20 pages each, then there are Service Manuals of the same length.  Some dont have copyright dates, others are 1950.    All very interesting.  Here is a pic of the booklet covering HVID engines.  There were many of this old type plant at a local antique tractor event earlier this summer.

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.