How much did David earn at Murdstone and Grinby?

From the text:

Mr. Quinion then formally engaged me to be as useful as I could in the warehouse of Murdstone and Grinby, at a salary, I think, of six shillings a week. I am not clear whether it was six or seven. I am inclined to believe, from my uncertainty on this head, that it was six at first and seven afterwards.