I have yet to identify the exact patents relating to the bolt-back design but I have found there to be at least five variations on the theme:
1. 4 part frame – curved – nut exposed inside (4-C-NE)
In this case the seat frame is rounded and complete. Bolts pass through the stile, then through the continuous seat rail to be secured by bolts which are visible on the underside of the chair.
2. 4 part frame – squared – nut lost in rail (4-S-NLR)
In the second case the back rail of the seat frame has been squared off, and a bolt passes through the stile and the edge of the back rail, then into the end of the side rail, where it engages a square-head nut hidden in a slot mortised in the inside of the rail.
3. 3 part frame – squared – nut lost in rail (3-S-NLR)
In the third case the seat frame has no back rail and the bolts pass through the stile, directly into the ends of the side rails, again engaging with nuts lost therein.
4. 3 part frame – curved – nut lost in stile (3-C-NLS)
In the fourth case, the bolt passes from the inside of the seat frame into a nut, mortised in the stile between the dowels which attach it to the side rail. the nut is thus invisible.
5. 3 part frame – curved – nut exposed inside (3-C-NE)
In this case the seat frame is in three parts with bolts passing through the stile and rail to a blots exposed inside.