Dave wrote:

It doesn't have two lock rings, just an adjuster to take out the
backlash in the screw thread and the ability to rotate the fixed barrel
that has the 1/10000 vernier scale on it. As I wrote earlier, they are
very stiff to move.

I was talking at cross purposes then, mine is a 0-1" and 1"-2" with two
separate anvils to change the range. The anvils each have two lockrings
with notches for a C spanner.

AJH