The rooms seem to be two trapezoidal chambers I have marked A and B on the attached plan (north at top; from Richardson, L., Pompeii. An Architectural History, Baltimore 1998, p. 404) whose west wall is on the same alignment as the main external arch of the Porta Marina (not the footway arch which protrudes further west).
The problem with this plan is that it doesn't give any indication of terracing, which makes it rather confusing. But if you go to Google Earth you can see the modern roof area covering these two rooms immediately above (north) of the Porta Marina and it makes it clear these are the rooms marked on the plan. Or look at Google Maps: http://maps.google.co.uk/maps?t=h&hl=en&ie=UTF8&ll=40.748594,14.483288&spn=0.000981,0.002403&z=19
What is not so clear is which house these rooms belong to. They could belong to the House of Umbricius Scaurus or to Insula Orientalis.1. They are on a lower level than the main floor of both houses.
However, if one examines the facade of the Porta Marina it does rather look as if quite a lot of this superstructure has been substantially reconstructed. I am wondering if these rooms as extant are in fact largely early modern, perhaps even preceding the main excavations, and built on Roman footings. The superstructure here is totally inconsistent with the very poor state of the House of Umbricius Scaurus immediately to the north.