I just purchased a 2013 KH-2 Vintage that didn't have the circle stamp. I was debating between that and a 2012 that did. I went with the 13 for 2 reasons...first off I felt the Sig Series circle stamp on the 12 was more in line with the SS standard series serial number where as the Custom Shop circle stamp was more congruent with the Kiso K numbers. In other words I felt that guitar was sending a mixed message, ha!
Also in the end I felt the lack of a circle stamp was more similar to Kirks actual KH-2 from the early 90's. I had to look back on some footage from Year and Half to find some shots of the back of the headstock and voila it was plain. So sounds like yours is similarly plain.