On a field of blue, the flag displays a yellow emblem of a crown and crossed arrows. This is the emblem attributed to St Edmund, king of East Anglia, shot with arrows and decapitated Vikings in 869. A shrine and cult later developed in Bury St Edmunds, and the emblem gradually came to represent the county as a whole.
A warning to the curious
http://gutenberg.net.au/ebooks06/0605921h.html
“So, if you find a crown / It’s best you leave it in the ground / Or sadly I will have to track you down / And turn your sweet smile upside down”
That’s a slightly different tale. M.R.James was very much in favour of the Three Crowns of East Anglia, which includes the arms attributed (much later) to the Wuffing dynasty which ruled East Anglia a couple of centuries earlier - and who seem to have come from Sweden - which also uses these arms - and based his tale on those arms.
Evidently a crown was dug up near the Wuffing palace at Rendlesham in the C15th and melted down - as in James’ tale - but, according to legend, Edmund’s crown was recovered soon after his death. The legend tells that a wolf guarded his decapitated head until it was found - still wearing the crown, according to most depictions, however improbable that seems.