D&RGW early F's were the same way. They were drawbarred together into four unit sets. For instance, 540A - 540B - 540C and 540D.
Which became, 5401, 5402, 5403, 5404, and then the numbers skipped to 5411, etc.
My uncle, who worked for the Grande in the mid - 60's, told me that drawbarred sets existed at the same time some had been broken up into "individual" locomotives, so, since there were such things as A units, B units, C units, and D units, nobody ever referred to an F unit without a cab as a "B" unit. Since it could also have been a "C" unit. Hence, they were always referred to as "boosters"...
The only railroad that did call them boosters as far as I know...
Robert