When I was building embedded controllers with 6502 processors in the 1970s and 1980s we used UV erasable EPROMS and a programmer (my own design) with a ZIF socket built on an expansion board in an Apple ][. The prototype board also had a ZIF socket but the production boards would have ordinary DIL sockets, our production volume was too low to warrant ordering actual ROMS so we used the UV erasable ones and put a metallised sticker over the window.

All programming was done on the Apple in 6502 assembler. It took 45 minutes to assemble an 8kB rom image. This meant that you took extreme care to think about what the code was doing as assembling a new image was often the most time consuming part of the Edit-Assemble-Test loop.