It's a 739. Older 739s have 18 FC seats while newly delivered 739ERs have 20FCs. They're assigning a 20-seater for tomorrow's 647, but they're only booking 18 in case of substitution.

FlyOA2 is correct about the situation with the 772. Same reason. They will only book 48, but the aircraft assigned may have 48 or 50 seats.