I don't even know how they are working with no igniter, but they are working.
Maybe the fixtures were originally HPS and were lamped with MH?
Probe-start MH ballasts do not have ignitors, but they are an older (though still current) tech. Pulse-start (new tech) ballasts DO have ignitors.
As to what the fixtures are actually supposed to be, check the transformer for the ANSI code... prefix M for MH, S for HPS. It's typically a letter and two numbers, like M47 or S55.