What Does SPQR Stand For?

The abbreviation SPQR means, in English, the Senate and the Roman people (or the Senate and the people of Rome), but what exactly those four letters stand for -- in Latin -- is a little less clear. My take is that SPQR stands for the first letters of the following words with "-que" added as the third:

Senatus Populusque Romanus.

That -que is added to another word doesn't mean it wouldn't be heard as a separate unit of meaning. Spelled out in this way is the inscription on a frieze on the Temple of Saturn, at the foot of the Capitoline, which may date to a restoration in the third century A.D. [Filippo Coarelli, Rome and Environs]. Many think the letters stand for Senatus PopulusQue Romanorum, which is what I thought until I realized that that would be redundant -- translating as it would as "the senate and the people of the Roman people". There are other variants, including Romae, instead of Romanus or Romanorum. The Romae could be a locative or a genitive. There is even a suggestion that the Q stands for Quirites in some form.
It seems likely that Romans used SPQR to stand for more than one of these options. What is your opinion? Do you have any evidence? Do you know of any uses of the abbreviation before the imperial period? Please post in the Readers Response to What Does SPQR Stand for or read earlier discussions.