It ends up meaning oats, and also a stretch inside, which is a fair bit of travelling for one small word. It started as pottage — anything thick and boiled, mostly vegetables, a bit of meat if the week had gone well — and then the middle of it softened the way spoken words do, through porrage and poddish and whatever your grandmother's village said, until it landed on oats and stayed. Rosa's salt-early rule is older than the oats part of the meaning, probably.
The prison sense came along in the nineteenth century, out of the food itself as far as anyone can show, and then a sitcom in the seventies with Ronnie Barker in it nailed it into the language so firmly that people now think the show invented it. I've a slang dictionary behind the counter, two copies actually, and only one has pencil in the margins — somebody went through the prison entries very carefully, and I'd love to know why.
A spurtle is just a stick, really — a thin rod of wood you stir porridge with so you don't bruise the oats into glue. Salt goes in early, that's the only rule I keep. And Mary's Meals have been ladling the stuff out since 2002, starting…
Salt at the start, yes, in with the water, because oats that get salted at the end taste like two separate things sitting next to each other. And I've scorched a pan of it — walked off for four minutes, came back to a smell that stayed in the kitchen till lunchtime. Pottage makes sense to me; anything thick enough to catch.
Jun, the sitcom's dates are fussier than people remember — the Fletcher pilot went out in 1973 inside Seven of One, and the series proper only started in 1974. (The slang was doing fine on its own for a hundred years before either.)