Have anyone an explanation? I took off a pair of old 906 heads from a running 383 and found this. Looks almost like someone has made threads in the valve guides, in all the exhaust, not intake...

Looks like they may have been knurled. Sometimes machine shops will run a knurling tool down the inside of the guide. The tool cuts into the metal leaving raised edges, which are then reamed. The idea is to make the insides of the guides a bit smaller, trying to make less slop between the valve stem and the guide, thereby cutting down on the amount of oil that passes through. It has been my experience that it only prolongs the life of the guide and lessens oil consumption short term.

That's a short term repair to get the valve stem to guide clearance within spec.

Knurling is pretty standard practice for a non-performance valve-job.

Thanks for the education!