O/4148/2 is the reference no. of the Silver War Badge list on which his name appears. You will find the list on Ancestry in the "UK, Silver War Badge Records, 1914-1920".
The "Medal" card is only for his SWB. He didn't qualify for any medals as he never served overseas - so his epilepsy was not due to shell shock!
16th (Transport Workers) Battalion of the York and Lancaster Regiment remained in the UK. During the time George Grayson actually served with them (31/8/17 to 24/9/18), they were at Durham and then Catterick. (He volunteered on 24/6/16, but was not actually called up to serve until 31/8/17, according to his record.)
392(xvi)KR is Paragraph 392 Subsection xvi of King's Regulations, i.e. discharged "No longer physically fit for service".
Adrian