Morris Minor Owners Club

Does anyone know what the numbers relate tooo that are stamped on block face. I have one engine that I am rebuilding and it has no. 1 adjacent to 1 + 2 bores and no. 2 adjacent to 3 + 4. The second engine has no 7 next to all bores.
Someone said they were grades. If this is true can you explain grades?
Ian.

It relates to the 'grade' of piston used, but I'm yet to find an explanation of how and why the pistons were graded