The consensus seems to be that SPQR means Senatus Populusque Romanus, but there is also the theory that SPQR did **not** mean Senatus Populusque Romanus.

It could also may have been **S**enatus **P**opulus **Q**uirites **R**omani.

I've read this in the entry for *Quirites* in the dictionary  *Langenscheidt Großes Schulwörterbuch Lateinisch-Deutsch* which I unfortunately don't have access to right now.


[1] http://de.pons.com/%C3%BCbersetzung/latein-deutsch/Quiritesque

[2] http://www.treccani.it/enciclopedia/quiriti/