It (BB shell ID) measures out at 33.0 mm (plus or minus a tad) so the closest match to that is BSC 1 1/4" 24tpi (http://www.sizes.com/tools/thread_BSC.htm) Brilliant. Bloody fantastic.
Walk into lbs and ask for a bb 76mm (actually 3") for the BSC 1 1/4' x 24tpi !
Chances are they'll understand the first two letter (bb) and then go rapidly blank.
Looking even more like a machining job.