I measured a ECG (272/292) rear seal retainer and here are the dimensions of the rear seal groove.
O.D. – 3.200”
I.D. – 2.720”
Width at O.D. of grove – 0.200”
Width at I.D. of groove – 0.240”
Here are the dimensions of the rear seal groove in the block.
O.D. – 3.200”
I.D. – 2.700”
Width at O.D. of groove – 0.190”
Width at I.D. of groove – 0.240”