Imprisonment for...

[Greensleeves]

I agree with Gadget; whilst to me the letter looks more like an 'S' than an 'L', it wouldn't be the first time that someone hastily making an entry was not entirely accurate.  'Leaving his guard' would be presumably a specific offence which would be punished severely because of the implications for security of the area.

[stanmapstone]

It is "Leaving his Guard" there are numerous reports in the newspapers of such a charge.
For example from the Leeds Mercury - Tuesday 14 May 1935
".....Training Battalion, Royal Signals, Catterick Camp, has been sentenced by district courtmartial to 56 days' detention for leaving his guard without orders, absenting himself without leave and conduct to the prejudice of good order and military discipline."

Stan

[mazi]

Well that has whiled away an evening, I now know that by the 1800s a dragoon guard was mobile infantry not cavalry.   

Mike

[Henry7]

Mostly we don't all always write in neat, copperplate handwriting, not even army officers.

And here's one such (possibly desperate to get away to the mess) having to write about some lowly Private who was given 82 days in clink, for leaving his post while on guard.  No wonder it's a bit scrawled.

[onyxandopal]

Thanks all. I'll go with 'leaving his guard' . 

[ShaunJ]

Queen's Regulations 1844: