Possibly some time up to ~ '77-ish?
The "signature" is in Marie's writing.
None of the "artist" signed pieces from Mdina are actually artist signed, apart from the odd one by Michael Harris.
Certain good peices were deemed good enough to merit an "artist signature", but these artist scribbles were added by Eric Dobson (mostly) or Marie.
We don't know what they're supposed to read.
This "three spirals" design was in production while Boffo was still there. It is found on chargers like yours, "bullet" bowls and I have a large Beaked vase, by Boffo with this pattern on.