The '73 cars with 4 wheel drum brakes had the small bolt pattern. '73 cars with disks in the front from the factory were BBP front and rear. If your '73 had both it was either a 4 wheel drum car that someone upgraded to disks, or a BBP disk car that someone took the BBP rear out of.
Probably the first one, because even the 4 wheel drum cars had the larger ball joint UCA's. So the disk swap just requires the disk spindles and brake parts.
