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Hi, and welcome to JA!
SPQR stands for the Latin Senatus Populusque Romanus (which means "The Senate and the people of Rome" in English).
The last question on the following page says that "slaves, gladiators, criminals, and later, soldiers, were tattooed, as an identifying mark." Basically, the Roman Empire marked their soldiers with the SPQR tattoo.
I hope this helps,