6B/234 is the Air Ministry stores code for a Mk VIII watch, which was of a lower specification to the Mk VII (navigational standard) watch, coded 6B/159.
The Mk VIII was issued for those tasks that didn't require the accuracy of a navigational watch, and ended up being a general aircrew item.
Interestingly, the US-supplied A-11 watches, as used by the USAAF for navigational purposes, were deemed not to meet the 6B/159 standard and were thus marked 6B/234.
Similarly, 6B/159s that, after repair, did not meet the original specifications were downgraded to 6B/234s, with the old code being barred out and the new one engraved below.
I notice your watch is either in a left-handed case, or the dial has been repositioned.
All the best,
PB