Do you have sight of the schematic diagram? It's a scary thing to look at, but I notice that the Red power wire, labelled 10 (Red with no tracer) originates at the 'Bridge Rectifier', towards the right in the lower half of the diagram, and is fed to playfield solenoids and relays, then changes to Red-white, 15, upon reaching contacts at the Tilt relay. So if that is the Red wire on the fragment, we can follow it through the diagram, where it moves along to the right then U-turns to reach the upper half of the drawing.
Looking at each device it's connected to, I don't see a Grey-blue wire used, but a couple of Grey-black. One is 'Alternator relay', the other 'Targets Sequence No.1'. Another look at your close-up picture shows a paper label caught in the wiring near the vacant relay mounting; I may be able to guess at the lettering 'Alternator Relay' on it. This may have detached from the relay bracket in an overheating situation that destroyed the relay.
Both of the two uses listed use the same relay winding, a G-32-2500.