Official Report Number tends to stand for internal numbers assigned by the University or Research Institution for Technical Reports. For example, ATT Bell Labs often published their internal research documents as Technical Reports, and the Computer Science department at M.I.T. also tends to publish internal research findings, often supported by government sources or sponsored-research funded by corportations, as "white papers" or technical reports.
MIT's library is pretty good at finding these things: http://libguides.mit.edu/techreports, and their webpage defines technical reports as
What is a technical report?
- Are written to convey new developments or final results of scientific and technical research.
- Are usually funded by government departments or corporate bodies.
- Deliver technical information to the funding organization.
- Provide a forum for peer information exchange.
- Are not easy to find
Also, a lot of research funded by the military or DARPA or the OSD in the United States has a Final Report as its ultimate end-result and the means of disseminating the findings and recommendations, rather than a peer-reviewed journal article.
It is a shame that have an educational e-mail address for affiliation gets you a bye for submitting to arxiv; perhaps even academics should need an endorsement prior to being allowed to commit an article to arxiv. The seed recommenders would have to have been planted earlier, anyway.