In addition to the hotshoe and shutter connections, the RF-603 also has a threaded (“screw lock”) PC connection at the squared off end of the body, see above. (In the world of photography, “PC” once meant Prontor/Compur and not “Personal Computer”.) The intent of the PC connection is to connect to flashes that do not have a hotshoe. My set of RF-603s did not include PC connection cables so I ordered some from eBay (search for “Male to Male Flash PC Sync Cable Cord for Yongnuo RF-603” – can’t get much more specific than that!). My order was placed with “Jiakgong” in China. These cables were less than $3 each so I ordered 4 cables. Shipping was only $1.40; seems like it took about two weeks to receive the cables. The cables were loosely packed with no documentation but seem OK and I gave positive feedback on the transaction.
The eBay cables indeed fit the RF-603 – first test passed. Next, I wondered if the RF-603 could trigger two flashes in the same way that worked for the RF-602. The trick with the RF-602 was to use one flash in the hotshoe and connect the second flash with the cable. I tested this configuration with two old Nikon SB-24 flashes, the RF-603 and the PC cables as shown below.