If, by extension, you mean that the anvil is set far enuf back from rod's end that the end can be rested on interior muzzle and the indicator zeroed against the anvil, your pix certainly confirm your method of correcting for deflection caused by the cantilevered wgt. of the gauge and gauge mount block. I also concede that the Manson Precision cannot be calibrated in this fashion as the anvil is a ball turned integral to the end of the interior rod. My homemade has a bearing ball set in a shallow hole in the side of rod as yours does and set back from the end as yours is so I could employ horizontally as you do and calibrate as you do. I don't choose to but I could. Bore a hole in the end of the anvil of the Brownell/Manson Precision, press in a piece of drill rod or what have you, and it would then have the necessary "extension" also. A minor dispute over feculous pendacities, no doubt.
jack
