Wright2626 pretty much nailed it. The filler putty is, and I'm guessing, one of the Squadron products. I've used his method, back when I was modeling diesels, and it works. For some "fillings", I've also used my own concoction to fill in the holes.
The concoction was styrene dissolved into MEK, Acetone or regular styrene cement. It was mixed in a small paint jar, glue jar, anything that was clean. Pour in solvent until the jar was 1/2 full. Then place styrene pieces into the jar until full. Close the top, and let it work for 30-45 minutes. You can shake the jar every now and then to help speed the process.
This had the advantage of being able to pull a light mixture of styrene and glue, that can be applied with a brush, a little deeper in to the "gunk" has a little more styrene, and can be placed into larger areas, and then lastly, at the bottom of the jar, is a thicker layer, as the styrene particles do sink. It a "down and dirty" filler for when you run out of the regular brand you're used to using at 10:00 on a Saturday night, and the LHS owner just left for a vacation and the store is closed for that time!
The "brush" application would be used for light scratches and such on a smooth car side, extra holes for grabirons that are slightly out of alignment, or around a filled in hole, that is still visible because the putty wasn't soft enough to get into those fine areas. It should be applied in thin layers, no matter using the top, middle or bottom of the jar. It only takes a few minutes, depending on solvent used, the evaporation rate etc, to dry, and after another few minutes, can then be sanded. Sanding/filing is recommended between layers, to keep the filler build up around where it is applied to a manageable level. Since the filler is basically glue, it will stick to anyplace it's applied to, on purpose or not.