A while ago I purchased a Thread Dial Indicator for the ML4. The saddle doesn't come ready drilled so it hgas been a process of checking the location then going through the fitting process.
TDI, bracket and fittings were purchased on eBay
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This is where the bracket fits and to mark, drill and tap the holes I dismantled the saddle top from the apron.
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Make sure the bracket doesn't rub the dovetail on the way
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Mounted the saddle top on the pillar drill clamped up to a piece of angle iron which is the best I can do to ensure it is drilled vertically. Centre punch marks just about visible.
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Tapping 4 BA. It was a blind hole which I started with a second cut tap and finished with a plug. WD40 as lubricant as so far it is all I have apart from clean engine oil in the oil can 
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Saddle top secured back on the apron and TDI fitted. Jobs a goodun 
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In dismantling the saddle I found that the 1/4/x 1/2 whit slot headed machine screws that fix it to the apron are a bit the worse for wear so I have some hex socket cap headed replacements on order.