As a follow up to a previos post by EDIpic converning the use of the letter "O for obsolescent" by the US Air Force (and Army).
This subject seemed to catch several people by surpise, and discussion ended with agreement that the practice ended perhaps in the late 1970s.
Just yesterday, I was at the EAA Fly-in at Glens Falls, NY, and there was an H-60 on display. It was typical of US Army or Army National Guard (ARNG) H-60s in that it had few markings of any sort. It did not have a state identifier on the tail, and in fact had only part of the serial number visible. This, with the exception of an aircraft name on the nose, was the only marking on the aircraft. Clearly visible in yellow, were the last three digits of the s/n, 442. But by walking right up to the tail of the aircraft, you could then make out the preceeding four characters, painted in black. They are shown in quites as follows:"O-24". This makes the full serial number representation of this aircraft O-24442, which according to my s/n books is likely to be aircraft
85-24442, a UH-60A. My guess is that is aircraft is from the NY ARNG at Albany. This significance here is that it shows the practice of the leading O- marking is still in effect.
I have looked in several older references for mention of this marking practice. Perhaps it was never understood correctly by writers in the past. That there are two related interpretations was brought up in the earlier topic.
One writer states that it is the "letter O, and it stands for obsolescence." Another writer claims it is the "numeral Zero, and it means the aircraft is more than ten years old." I believe that today it is a zero, and it means the aircraft is more than ten years old.
There are many ARNG H-60 units. Perhaps someone with a copy of the applicable regulation can step forward and give us the definition of "O-" as it is presently used.