Late 60's were transitional years for those cabs. A '68 big Bassman cab had 2x12's for a while, then moved to the 2x15's, still leaving a good bit of room inside. The move on the Bandmaster and Bassman expanded cabs were for looks only, trying to create a much larger appearance for competition with some of the larger cabs being introduced by other companies at the time. When the Bassman went to the 2x15's, the Bandmaster stayed with the 2x12,s and also had 4x12's. All 3 cabinets were the same size - 39" tall, 28" wide and 11 1/2' deep. Many unmolested cabs from then have a date stamped in black ink along one of the unpainted uncovered support wood pieces. My Bandmaster has F090769, indicating Fender September 7, 1969. An always there indicator for Bassman/Bandmaster is the location of the screw down receptacles on the top of the cab. Recall that the Fender heads had little wing like extensions that extended out to these areas and a thumb screw tightened the head down to the cab in this manner to keep it from vibrating or falling off. The Bandmaster distance from one receptacle to the other is approximately 13 1/2 inches. The Bassman distance is approximately 23 1/4 inches, since the Bassman heads were a good bit wider. That's the significant way to determine Bassman from Bandmaster.
I'm still experimenting with the Bandmaster cab and I've just completed the last few items on my '68 Bassman cab. The Bassman cab is a factory labeled JBL D140F cab, which I've found to be fairly rare. Long story short, it was a real basket case when I first got it well over 10 years ago. I've rebuilt and replaced on it since then and it's now totally finished. I can compare the Bandmaster and the Bassman side by side. I'm using a new Kustom KXB 500 watt 5lb head. For bass guitar, The Bandmaster with the Black Widows sounds real good, but the Bassman with the D140F's sounds even better. I can gig with either one of them though. Price wise, BW's would be a good bit less expensive to get. Anyway, hope this helps, at least in cab identification.