Difficult to work out why there are these two differences. Mann, Crossman & Paulin merged with Watney in 1958 and I think both labels are from around this time. The label on the right has the correct apostrophe for the Brown Ale from Mann. However, it may be that the label on the left is part of a marketing campaign to identify what was Mann’s Brown Ale as a Manns in a similar way that we refer to a domestic cleaning appliance as a Hoover. Or perhaps it was issued after the ‘merger’ and so it was no longer Mann’s.