Tyres (again!)

because the wear bars are generally 1.8mm not 1.6mm possibly?dont think tyres have compulsory wear bars indicating the central 3/4 either,then you have to prove that summat that can be on the limit when worn smooth(remember that some tyres have parts designed to do precisely that and you have to accept that as per the manual)are now worn below the limit.good luck with your idea but I think its a non starter.just use a bit of paper,simple,easy,quick,cheap,repeatable in the event of an appeal.....

But why dont all tyres have the wear bars this far out? if thats where we should be checking?

There is a make of tyre that is mainly found on 4x4's that when worn down to 2mm is a slick with two 8mm wide 2mm deep grooves in it.
All perfectly legal and perfectly safe in the rain ...... NOT !!!

The groove mark has highlighted is still testable to some degree where it falls into the central 3/4 ruling.
It contains a TWI which means it's part of a primery groove anyway,therefore the tester will have to take that into consideration.
I can see marks point that having a TWI in that position does help to identify primery grooves,but as he's said and we all know that not all tyre manufactures use the TWI in this way (section of tread) is purely down to them and they design possibly or production costs I can't answer that.
Primery grooves with TWI don't indicate the central 3/4 ,other grooves without TWI but are cut as deep are still testable remember and need consideration under the 3/4 ruling .
I have noticed that Michelin are putting TWI in grooves on the outer shoulder

Just to back up what castrolrob has said really, we need to ensure that we don't misunderstand the difference between wear bars and tread wear indicators.

Some tyres have wear bars between the tread grooves, these are normally chunks of raised rubber that look like a tread wear indicator but are larger, to establish which is which, look at the outer/inner walls of the tyre at the edge of the pattern, where it meets the wall, you should see TWI marked on the tyre, if then you look into the grooves at the same line of action those bars between the treads are "tread wear indicators" TWI, any other bars will be placed elsewhere in-between the treads, if fitted, and are not 1.6 mm.