I'm gonna suggest that they're not actually "meant" for a brace, but 
likely mean for a production tapping machine. That notch that encircles 
the shaft just above the actual tap portion likely is how the tap is 
held into the quick-change chuck of the machine. The tapered square 
drive just gives it a quick way to be chucked and provide positive, 
no-slip torque.

Anyhow, that's my impression of what it's actually for.

- Bill T.

On 6/19/2012 9:33 AM, Caron, Gary J (PA83) wrote:
> Got about 40 of these at a local auction for the princely sum of $4.  They
appear to be bottoming taps which fit a brace.  Very fine pitch. 36, 40, 50, 56,
etc.  This one is marked 36   12,   which I take to be 36 tpi on a #12 size
screw.  Any ideas as to purpose?  And yes, I know my phone takes evil pictures.
>
>
http://galootcentral.com/index.php?option=com_copperminevis&Itemid=2&place=displayimage&album=random&cat=10160&pos=-6039
>
>
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