Bleed nipple 3.4S

[dsbiggles]

Anyone know what size spanner fits the bleed nipples on the rear calipers? Trying to sort with the IRS in situ and can't see exactly what's happening up there. It feels smaller than 11mm and bigger than 3/8. Also tried 3/16W and that doesn't seem to be right.
Of course, it may be non-standard if it's been changed in the last 50 yrs, but I might as well try the correct size first!

Concerned that one of the nipples already feels a bit rounded off, so any tips on something that will grip it better than a ring spanner given the, er, access available!?