'Sterling silver (the .925 marking) is a copper alloy, with some other metals mixed in.'
I'll give you the benefit of the doubt that you just made a typo, but for anyone else reading this, Sterling silver is 92.5% pure silver and 7.5% pure copper. This results in a more durable metal than pure silver and it also has a very slightly 'coppery' color when compared to silver plate.
It appears to me that the grips consist of a fairly thick sheet of Sterling silver, formed into the needed shape and with a reinforcement soldered across the back. The front is hand-engraved. The gold work appears to be thinner sheet gold (but certainly not fragile gold leaf) that must have been soldered to the silver. Possibly two gold alloys were used, 10K and 18K - unknown why, but 10K is very durable, having a slightly more orange or copper color than 14K-18K. Maybe the center plate with 'El Toro' is 10K and the 'ferns' of 18K - just a guess.
Anyway, I'm inclined to think those grips have more value than the precious metals they are made of. Could you tell if there are any signs on the gun that the grips had been on it a long time? I'm sure there is a story there but I doubt we will ever know it. Nonetheless, if the 1918 pistol is good enough to warrant a good set of original grips, you could sell the silver ones to pay for them.