I seen silvers drop on POD kills instead of elemental group. Never seen a elemental type slayer, but haven't done many of these. In a sample size of maybe 10 only seen silvers.
Good to know. I know the boss monsters were specially created for UOR, so loot tables were specifically created for them as well. Silvers are the "default" slayer weapon type that drop unless coded otherwise. This is one of many boss monsters that are specially created for UOR that need a slayer weapon drop adjustment made to their loot table. @Telamon has been correcting them over time as they have come up, this will hopefully get adjusted in time.
I think silver might drop on all MOBs and did prior to the other slayers being implemented GMs will have to confirm
@Zyler is correct, this is most noticeable in the drop rate of silver instruments. Have you ever farmed anything that didn't eventually drop a silver instrument??? Instruments and weapons use the same system to calculate %chance loot.