Sorry for delay in responding, for some reason I didn't receive the usual reply notification about your message.
Sounds like a "silver war badge" card. The cause of discharge the A.O. Number is the Army Order authorising his discharge, the "pass" or "poss" is actually "para" and I suspect the the 14 maybe is 17 so the whole thing becomes AO256/17 para a2 [or 2a] which I think indicates "unfit through sickness", but that will need confirmation. It's possible there is another card or it's missing.
If he enlisted August 1914 he may well have served in the 11th [service] battalion, one of the first new army battalions formed in response to Lord Kitchener's appeal for 100,000 volunteers [K1], in which case he served in France from 29 May/1st Jun 1915 see http://www.1914-1918.net/msex.htm
,but again identifying the battalion will need confirmation and I'm afraid that can only be done by looking at the Middlesex Regiment medal rolls.
Your reference to the 29th Middlesex can be slightly misleading, the 29th was a "works battalion" and wasn't formed until 1916, 2 years after William had enlisted, it was made up of men who were no longer fit to serve as 1st line infantry at the front, either through wounds, sickness or age; they transferred to Labour Corps when it was formed in 1917.
You may be interested in this site http://www.geocities.com/labour_corps/
and you may wish to contact the author.
The L.C 1031 is the Silver War Badge Roll reference number LC being Labour Corps' badge roll.