Quoted from rockrand:
I don’t know what is on your drops but they should be clean from anything other then what they put on the disrupters so they can be seen by the optos,I use black paint.
It looks like some kind of very thin grease. This game was NIB as well, so it wouldn't have been anything that a previous owner would have used. Again, it was a very thin grease like film on the back side of each drop target, not just on one or two. I had made a mental note of it, but did not bother when I put on the new ones as I had not used any grease on the back of drop targets before, but since putting on the new clear ones, I do find the far two drop targets somewhat "bricky" now, and I'm giving contemplation to putting some form of lube back there to encourage them to be more responsive. As mentioned by a pinsider above, might opt to use just a very light touch of super lube on the back of the drops, unless anyone else has any other suggestions.
By the way, what did you put the black paint on, out of curiosity? For these clear drop targets, is they're in area of the drop target under the play field that needs to be made black / opaque? I wasn't aware of that, so if you could expand on that, that would be much appreciated.