Concentric Circles on Subdials

The concentric circles in Speedmaster subdials has come up.

Are they on all Speedmasters?

At first I thought that they were not on all Speedmasters so I had a look. They are clearly present in late watches:

Circa 1995
Circa 1995. Smudge near 6  is on crystal.

So these are clearly visible.

So I looked at 2998’s. Here is a 2998-61:P8040005

I do not have access to enough raw data so I can only present observations here and hope the wider community can contribute knowledge, perhaps via www.omegaforums.net

Here is an early 2998. They are hard to see in hand, but on inspection of a photo they are there:

P8040008  17761171 2998-2 CR

17301102 2998-1 CR

And here on some 2915’s. Note also the shallower sub dial impression on the plate.

2915 CR

P8040005-001