Stoneware jug marked ST or SI mark

I wonder if anyone could help identify this stoneware jug found in my mother's house.  She lived in Suffolk but not sure if that's any indication of where pot is from.  Lovely treacle brown glaze, sturdy pulled handle with what looks like an ST or SI in rectangle at base of the handle.

Do you have a larger photo of the mark? Seems to have uploaded thumbnail sized photos

I hope this is a bit bigger?

A smidgen bigger. But a bit more visible