They're Die letters rather than mintmark, but it does look like DA.
This site has some information, although it doesn't mention DA. http://www.wbcc.fsnet.co.uk/sub004b.htm
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I'm not an expert in any kind of coins, but I reckon I'm good at research and will do my best to help. Feel free to tell me my identifications/valuations/gradings are wrong. It's the only way I'll learn.