If you go to Christine's suggestion and start trawling - Mss. and Special Collections > Accounting > Bills and Receipts > Receipts, Vouchers and Quittances > Bills and Receipts for the Marquis of Titchfield > Enlarge p.1. The sum at the bottom is quite clearly written £50"15"6, although the "quotation marks" are at the bottom, not the top. I would think this is a clear indication that at some point these marks were generally used just to separate the pounds, shillings and pence. In your case probably just the pounds from the shillings because there were no pence involved. There might be even more examples if you trawl around. Hope this is useful.