make a standard c7shape,younow have c79,moveitup twofrets youhave adifferent d7,move it up again two frets you have e7.
half bar your top fourstrings on second fret=a major chord dont play six string,you can play in amajor,move this up two frets youhave b7,dont play six string.
play your standard e major chord you now have another e7,yoursix string is also a seventh,but I tend not to use it.Dick Miles [ concertina player]