The North Wales Weekly News Thu, 22 Jul 1915 ·Page 6 has a list of Penmaenmawr men serving. It includes Thomas J Williams, Castle Hall, 17th RWF.
17th RWF are linked to Llandudno thus:
17th (Service) Battalion (2nd North Wales)
Formed at Llandudno on 2 February 1915 and came under orders of 128th Brigade, 43rd Division.
Record same as 15th Bn but not disbanded in 1918.
Given the Absent Voters shows him as a Welsh Regiment man it appears he has either transferred to the Welsh Regiment at some point before December 1915 [which is when he went to France]
OR
He was discharged from RWF having been found to be under age and has then enlisted in the Welsh Regiment making himself older to do so.