I suspect you mean Coventry Easy Change magicniner?
I`m adding a sketch i made today With all Critical measurements.
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Click image for larger version.  Name: EasyChange image.jpg  Views: 779  Size: 96.8 KB  ID: 20034
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Will also send it to Coventry Toolholders and have them confirm all measurements and explanation as to why the groove is missing in the top flange in their catalog.
Believe the flange is a special Boxford solution on their first ATC`s.. or something...
Would be interesting to know if there`s anybody that would buy any of my bearly used standard Easy Change toolholder like the on in the Picture though.
I`m more into collet chuck toolholders and the Shell Mill Holders.