I’ve read many times that people analyze their pinholes with a scanner. I tried this today with Epson V600 and my conclusion is that the resolution is nowhere near to make any good analysis. The shape / quality of the hole is probably pretty hard to see. I scanned at 2400 dpi and on my file one pixel equals 0.01mm so the difference between 0.28 and 0.32mm pinhole is only four pixels.
These are two commercial pinholes from eBay. The millimeters are the advertised sizes and the cyan lines are lines placed for the advetised millimeter distance.
Because the resolution is so low even at 2400 dpi scan, I can only say that the pinholes might be as advertised based on the measurements I made. Both pinholes were purchased from same seller (1 year apart). Based on sellers microscopic images the hole should be really clean.