"Roman Soldier: Give us a Martinus"
Barman: "You mean a Martini"
Roman Soldier: "If I want a double I'll tell you"."

This is actually from "Rinse the Blood off my Toga", and it is not a Roman Soldier, but (Lucius Maximus?), the Roman Private Eye - by Wayne & Schuster, available on CD - web references abound.

Another classic from the same source - but it's a visual joke - use your fingers to count -

"I put 2(II) & 2(II) together, and it came out IV"...

:-0