Streetmap: 'mX', 'mY', and 'M' ??

Hi Guys,

[Newbie alert!] -- I've used streetmap.co.uk to get some location data. On
their coordinates page for a specific UK postcode, an instance of which is
linked below, they have OS x and y, long/lat, and 'LR' which is the
Ordnance Survey Landranger Grid Reference. These are followed by 'mX',
'mY', and 'M', I've no idea what these are and am very curious. Maybe
someone in this group will know?

http://www.streetmap.co.uk/streetmap.dll?GridConvert?name=450679,208578&type=OSGrid

Thanks all.