6203 is a standard bearing. The z refers to the seals. Z iirc is open.
If I'm understanding correctly, 6203 is arbitrary as I don't see a connection between these numbers and the size. The 'Z' apparently is sealed on one side whereas 'ZZ' is sealed on both sides.
Update: After looking, '03' represents the bore size which is 17mm. Think I found the correct part number break down on the NTN site.
The upper shaft in the drill press has been removed and the bearings are back-to-back, so their inner sides may be open (I'll press them out to confirm), but I can't see. The STEP file for a 'Z' bearing has one side open, and initially I assumed they did this to show the bearings, but a 'ZZ' CAD has both sides sealed. Plus one of the PDFs supplied in this thread seem to confirm a 'Z' bearing is open on one side (not sure why someone would want one side open).
Since 6203 defines the bearing size, what other factors should I consider besides sealed on one side versus both sides? I tried to follow the PDF on bearing quality, but it looks like ISO 492 has classes that are tied to dimension tolerances only.
My assumption is I need to consider quality, grease, and seal type. Since they probably get more expensive, and this is only a drill press, it probably only needs some basic bearings.
Update: that PDF of the NTN catalog actually somewhat outlines the things to consider and I see LLB (also LLU that looks better).