Jack,
I'm going to go out on a limb here and say that it is a QC-54 (10") because of the two-piece gear guard and nearly square shape of the larger bottom portion and the three hinges on it. Plus it doesn't have a motor belt guard. Plus the inverted-J shape of the countershaft bracket. If the QCGB was a retrofit, it could also have Model Number TH-54 still on the nameplate. With the QCGB, it will also have a tumbler style reversing mechanism instead of the reversing gearbox found on the Change Gear 10"s.
The Countershaft Bracket on the 101.27440 or any of the earlier 12" that you could retrofit a QCGB to (the other possibilities) is almost straight.
Robert D.