From scubaengineer.com:
https://www.scubaengineer.com/pictures/scba_cylinders/luxcompinspectmanual.pdf "
5.4 Internal Inspection
a) Reject all cylinders with internal isolated corrosion pit(s) estimated to be more than 0.03 inches
(0.76 mm) deep.
b) Reject all cylinders with sidewall line or broad-spread corrosion when one or more interior
pit(s) in the line corrosion is deeper than 0.020 inches (0.51 mm), and/or if the interior broad-
spread corrosion is deeper than 0.020 inches (0.51 mm).
c) Reject all cylinders that have bulges or dents on the inside of the liner. This indicates severe
impact or another form of serious damage."
The thread depth is around 0.7mm and the roughness of the inner surface is nowhere
near a) and b). Passes (c) as well.
So, I guess some roughness is allowed even when corroded in someway.
Now I am very curious to see the inside of an FX bottle.
If we do not have any photos, everything else is speculation.