hello Richard,
here is the link with the micro films rolls :
http://www.airforcehistory.hq.af.mil/faq/MACR.htm.
the serial numbers of the vol2 are from 686968 to 1085953, so 888602 should be there.
In fact two wrecks of machine guns have been fished :
one with the serial readable : 888602, with what seems to be an automatic system of armement,
the second part of MG , in very poor condition, serial not readable, has a manuel system for armement, so man handed at a window of the a/c.
A part of frame, also in very poor condition, seemps to be from a B17 and not from a Lanc or Halifax.
Wise62