Need DOS 3.3 to restore a PC based instrument

Hello all, I have just acquired a PC-based instrument. It's a Model DTA 8850 Digital Timing Analyzer, made in 1989 by a company called International Test Instruments, Inc. in Irvine, CA. Unfortunately, they seem to be out of business. The machine is based on a PC architecture, with the specialized signal conditioning circuitry on a backplane. It has some sort of non-volatile electronic disk as the C drive. I was able to determine that the operating system is DOS 3.3, but there is something munged about it, because it won't run the application software for the instrument. I can boot the instrument from a floppy, but the only version of DOS I have is 6.22, and the instrument gives "Incorrect DOS Version" error messages when I run the application software (but it does run). I would like to get a copy of DOS 3.3 to restore this machine to its original state. Also, does anyone know anything about International Test Instruments, or (fingers crossed) have a manual or other information about this machine? Thanks in advance, Stan