Carb rebuild kit number

The list number you have found is very likely 4285B, not 42858, but I can't find either. 4656 may be the actual list number; read through this thread discussing list number locations and kits for this carb: http://www.slantsix.org/forum/viewtopic.php?p=234199

I can find a kit on Mike's Carburetor for the 4656 but not for the 4285B.

And the last time I looked for a 1920 kit a few months ago, the best listings I found was at Autozone, believe it or not. The accelerator pump shaft is one of the variables and sometimes you need to take the parts to the store and compare that part with what is in the kit. Mike's Carb kit are pretty good and include a few extra parts that others do not, and they have good videos. Mike's is the only pace I know that has new 1920 floats. Walker kits are a very often recommended brand.