if the thread spacing is tricky to measure individually

either count how many threads in 1" 25.4mm for tpi

or measure the distance for 10, 15 or 20 threads. write that down, then measure the distance trough to trough and divide by 10, 15 or 20

and if you still think it's whitworth

http://en.wikipedia.org/wiki/British_Standard_Whitworth

machine shops should have pitch gauges, if you know someone with a tap and die set they often have pitch gauges too

my old nacra apparently has a strap of brass set in the glass acting as a "nut"

alloy would probably be more likely than stainless as stainless bolts going into stainless threads in a marine enviroment sounds like trouble

tapping a new over-size thread into a alloy strap should be dead easy