Swords & Knives (also: spies and jam)

( In which I am Baffled by 4th Century Iron Things.  )

Skipping back a couple of centuries, I am intrigued by Hadrian's Frumentarii secret service, but wish to put a cherry on the top.  Would it be ridiculous to invent a Senatorial secret service working in parallel and sometimes at cross purposes with the Imperial one? 

In other news, I am unconvinced by rhubarb jam. It doesn't seem to be very... jammy. It is more like a pie filling in a pot.