Thanks to all for sharing your knowledge.
Rod222 very much appreciated, I searched for a full day and came up with nothing.
So many sites with great info it's hard not to get steered away from what your looking for though.
I may send photo's to the one site above and see what they have to say.
It seems that the dot may very well have a purpose but nobody has determined just what that purpose was.
I also added a photo with a measurement for added info.
I don't see any sheets or corner blocks without an inscription on e-Bay, were there sheets without the inscription also?