The lower portion of hold #3 was a "bunker hold" designed to contain 1,092 tons of coal for use in the ship's boilers. Stokers and trimmers could access this fuel by going through the watertight door closing bulkhead D and into a small vestibule. From there, non-watertight "man doors" led into the bunker hold. These two doors meant that the so-called "black gang" did have easy access into the lowest portion of hold #3 directly from boiler room #6.
However, getting into hold #3 and accessing the mail or baggage storage rooms on the Orlop would have been another thing. It would have been necessary to scale the trunking of the bunker hatch and hang by fingernails while opening doors secured from the other side.
Looking at that small vestibule, the after watertight door sealed bulkhead D. A second watertight door (centered between the non-watertight bunker doors) actually served to seal bulkhead B. The fireman's tunnel was effectively an extension of bulkhead B pulled aft right through bulkhead C. There was no closure in the tunnel at bulkhead C.
-- David G. Brown