answer

A reader, Roger asked me this morning about my boring machine
this thing is a gnarly beauty and built like a tank
everything on it works perfectly, though it's obviously lived a long hard life
there are no maker marks
though a little research I figure it's Shaker made between 1850 and 1900
-edit- Roger tells me most likely post 1870, and likely only used by the Shakers -edit-

the clasp at the top appears to be part of a mechanism attached to the main casting, the long lower arm extends to the bottom of the spur and ends in a simple hinge, and a small spring is fixed in between

 

on the cross piece above is screwed a beefy brass plate as a catch

to be honest, I wish I could find a project to use it on, guess if I add a shed some day I'll just have to do it timber frame:)

the humdinger for me involved to wood shelf out front, was that to aid in clamping it down?
there are no marks whatsoever that look like it was touched by a clamp of any kind
then I found this: near the end of the page is a perfect example of the Shaker butt vice
man those guys were smart

there are actually bits for sale online, ranging from fancy new ones to refurbished and sharpened antiques to rusty old shed finds