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I bought these Ancistrus two days ago and I was told that they are Starlight Ancistrus, but the guy that sold them to me wasn't sure what the L-number is. He says that their father is about 14cm big.

Welcome to PlanetCatfish!
Ancistrus are always difficult to definitavely ID without any collection details, but a quick look through throws up (also known as L182) (which has a common name of Starlight Bristlenose, but is different from the other Starlight , also known as L183).
If these came from a shop, next time you're in there, ask if he knows where the fish were shipped from - it may not help but it will at least eliminate some of the possibilities.
I'm not sure it is L182 from the shape of the head, but there are that are much more knowledgeable then myself who will hopefully chip in.
HTH
Martin

The one thing I forgot to mention was that when the fish were young the apparently had white lining on the fins...

I'll have to ask him if he knows where they were shipped from next time I chat with the dealer.

FYI L183 keep the white seams into adults (I have 2 of these) and they also have 10 dorsal fin rays, the only ancistrus that does as far as I know.
Martin

I asked the guy that sold me the fish, but he bought the parents about 4 and a half years ago in a local fish shop. Thus we have no idea where they come from. I tried counting the dorsal rays, but these seem to have seven. I guess that rules out L183 then.

Ok. I managed to count the dorsal rays. One hard seven soft. That is if I count it in the correct manner