It means only that he arrived in France on 11 September and, as I pointed out earlier, 1st, 2nd and 3rd Battalions had already been in France since mid to late August so which battalion he joined is not known.
My post about reinforcements was the result of a quick first reading of the 3 battalions war diaries looking for reference to reinforcements a little after 11 September. The battalions at that time had been heavily engaged in what became known as the retreat from Mons and in mid September were equally heavily engaged on the Aisne. The war diaries cannot, perhaps understandably, be totally relied upon to show when reinforcements arrived, what I quoted was from that first quick reading where the only references I found were to 1st Bn on 20 Sep (about right) and 3rd Bn on 4th (too early).
It may be that the next higher levels of command, the brigade and divisional diaries, may have more clues but that is all they will be, clues, there will be no list with his name. I'll take a look.
You either have to take a leap of faith and assume one battalion or another or, as I have said more than once before, apply for his service record which should show for sure who he joined in France.
MaxD
