Re: OK I got it, now what am I gonna do with it.
I looked it up (stolen post):
<span style="font-style: italic">Named for Dr. Franklin W. Mann, the foremost accuracy experimenter of the early twentieth century, these were used mounted in V blocks at those collars (usually two) with barrels of various diameters and characteristics to obtain results in work on velocity, accuracy, firing pin indent, and assorted other ideas with the Springfield 1903 rifles built during the 1920's and 1930's. Shorter barreled versions were constructed using the same receivers to do pistol ammunition testing as well. Apparently most any barrel could be configured to mount in the setup and extensive testing was done in those days.</span>
Interesting.