Getting farther along in the teardown (got sidetracked solving a bad cylinder problem). Anyhoo...here's the head tube setup.
The steer tube is ~25.8mm (+/- 0.2mm) at the upper and lower bearing. Isn't the stock airhead bearing 28mm ID...? My micrometer isn't perfect, but it's not that far off.
Any thoughts on what's been done here?
EDIT: I found the specs for an XR600 bearing -- 26mm, top and bottom. So, that's what I have here. Rather than an airhead steer tube pressed into the triple, I have a stock XR600 triple, with the head tube on my frame having been modified to fit. Good to know, should I ever need to replace the triples or the bearings...