Interesting comment to the article posted by ex forum member Edgar Brooks, Rest in peace Edgar.
Papers, in the National Archive, at Kew, give a few answers to the queries. The XVI, as a Mark no., did not exist until August, 1944, when it was realised that separate listings, for spares, etc., were needed, and the Air Ministry finally unbent, and acceded to the requests for a new Mark. Since other Marks had been introduced into production, in the meantime, it explains the wide gulf between the numbers. It's also the reason why it's impossible to find mention of the XVI any earlier; L.F.IX was the usual designation, whatever the engine.
All low-back aircraft had to have the "E" armament, since the planned fuel tanks, behind the pilot, entailed the relocation of the compressed-air bottles, which went into the no.3 Browning's compartment; at the same time the extra fuel meant longer flight times, so three oxygen bottles became necessary, two of which went into the no.4 gun compartments.
Another reason for the delay in the introduction of the XVI/low-back XIV was the reluctance of the Air Ministry to replace 4 x .303″ with 2 x .5″; it was found that, from the rear, the .5″ had no extra penetrative power over the .303″, and the general (lack of) shooting ability, by the average pilot, meant that the hosepipe effect of four guns, in a deflection shot, had a better chance of disabling the enemy pilot.
The arrival of the gyro gunsight changed all that, since the pilots' aim improved beyond all measure, so the A.M. finally went for the "E" wing. There was a further delay to the low-backs, though, since the electrical boxes, for the G.G.S., had to be installed before the fuel tanks, otherwise the tanks would need removal, for the sight to go in. All of this is the reason why the low-backs did not see service until 1945.