I have a (scan of) an 1858 volume of papers drawn from Sessional Papers of the British Parliament.
These appear to be Command Papers, which, according to various sources, were numbered  to  over the period 1833–1869. The Command Paper numbers (in 1857/8, in the low 2000s) do not appear in the papers, but they do appear in the index: e.g. here and here.
All the sections in the volume have numbers printed on the first page at the bottom left. Sometimes, this is the sessional paper number, eg. "155—Sess 2" (which agrees with the index).
But, sometimes, the number is a mystery to me. The section on page 345 is numbered "", which does not correspond to paper 120 in either 1857 session (1st, 2nd) - this section is, in fact Command Paper , listed in the index as coming from the 1857 1st Session, Volume XII., page 579. It seems this number is not a sessional paper number, command paper number or page number.
So, what does  mean in his context?