Thanks for the reply, Dave. I just looked, and my airframe doesn't have the numbers in that location. I removed both sets of shoulder harnesses, but the mounting tabs welded to the fuselage have no markings. The fuselage is powder coated, but there's not even a hint of anything under the coating.
The previous owner told me that he was about to buy a Model IV, but was encouraged by SkyStar to wait a bit and get the soon-to-be-released Series 5, which he did. So, I believe this is a very early Series 5. Perhaps they didn't start marking them the way you described until later, or maybe this is something that was started with the current company?