The 2N3771 on the back IS the crowbar transistor.
The 3771's are pass transistors - the crowbar is an SCR located inside the supply.
There is a 3771 on the back that drives the shunt SCR.
Better check the RS-50 schematic: there is one 2N3771 in the rear that is used to drive the 8 pass transistors - 4 per heat sink mounted on each side of the supply. A 2N3906 small signal transistor on the regulator board sets the crowbar threshold voltage and drives the SCR.
** To KD7YZ: If in fact you shorted the case of the single 2N3771 mounted at the rear to ground (the power tranistor that is mounted to the chassis and not one of the side mounted heat sinks), then you likely caused CR1 and/or CR2 to fail - not CR101 or CR102.
CR1 and CR2, along with capacitor CR6 provide a dedicated low current
DC source for the regulator and drivers, Q2 a TIP29 and the lone 2N3771 on the chassis which I refer to above.
CR101 and CR102 provide a dedicated high current DC source to the pass transistors which should not be involved, unless CR1 and CR2 failed shorted, causing the LM723 to fail, then the drivers and possibly the pass transistors - but I highly doubt this occurred. This would also explain why the fuse didn't blow which is rated to handle the full output current of the supply plus margin for start up current, etc.
One step at a time - isolate CR1 and CR2 and check them using a DMM with a diode test mode or use a Simpson on the X1 ohms range.