I just booted up one of the two 33mhz LCIIIs I have lying around here, as I am thinking of trying to sell it on. The hard drive would not cooperate at first, but it booted after a couple of tries with a welcome screen stating, "Welcome to Macintosh" and a second, unfamiliar line in the box, "Debugger Installed," and to a machine that reports itself as running System 7.3d0 in "About this Macintosh."

Never heard of such a System. What do I have? The machine seems otherwise just to have a 7.1 install, so how does it come to be ever-so-slightly more exotic?