Whitworth? thread identification

[Piek]

I recently bought a 4-jaw chuck to use convert to a welding table. No manufacturer ID nor any clue as to which machine it belonged. The jaws have a thread on the back and are fastened with a nut. I am missing a nut and will make a new one. The outside diameter of the thread measures 17.4 mm (11/16") and has 11 tpi with a 55 degree angle by the looks of it.
The spindle mount o.d. measures 35 mm (1 3/8") with 7 tpi.

I checked BSW, BSF etc, but cannot find a match. I can reproduce what I have but am curious if this is just an oddball or a standard thread I have missed?

 

[gustafson]

BSPP has an 11 pitch

just an oddball thread it sounds like

 

[Laverda]

Looking in my thread book, 11/16" x 11 TPI would be a "Sharp V Thread". An obsolete thread form as it is easily damaged. I have only ever seen it in 60 degrees but I suppose the UK may have made it in 55 degrees.