Master was the term used for a Chev car series in the 30's and 40's, for trucks it'd be like "AD" or "TF", covers a range of years and sizes .... this tech tip indicates the BL is an early 42 3/4T, with the 9 indicating it was made at Norwood, the 4990 is likely the sequence number

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics