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His pension card shows that he was also in the Labour Corps (service number 45319) -- this number was in a block (394500 to around 484900) that were issued between October 1917 and January 1918.

Allan. I'm afraid that you may have possibly misread his LC number. It is a 5 digit one and not a 6 digit one.
I now don't have access to a copy of No Labour, No Battle  to look up the 5 digit number but it's likely he was a KLR labour battalion man who went with the rest of that battalion to form an unit in the newly formed Labour Corps. I stress that I don't know for sure only that the number is low enough to fit that scenario.

There is a Frederick Victor Newick in the Leicester Absent Voters at 8 Victoria Avenue [Leicester South Polling area] He is shown as 141879 Gunner, 301 Siege Bttery RGA
Two other Newicks at the address are William Henry and Louis Wilfred Gladstone
Findmypast link   https://search.findmypast.co.uk/record?id=BL%2FER%2FD11%2FFMP000002381%2F0670&parentid=BL%2FER%2FD11%2FFMP000002381%2F0670

I wonder if Castle Hall was a name given to an area as Penmaenmawr increased in size?  Thank you for the census return, I do currently have a FindMyPast subscription so have downloaded it.

I have rechecked the Absent voters and it seems to be a house name on Conway Road, Penmaenmawr.
List goes like this
Castle Hall
Norwood
Cartref
Burleigh House
Bryn Gwynedd
Minydon
Bryn Dinarth
Bryngwynedd Cottage
Bryn Hafod

Here are Norwood and Cartref next to each other on Conwy Road on Google Street
https://www.google.co.uk/maps/@53.2708113,-3.919492,3a,66.8y,250.05h,89.03t/data=!3m6!1e1!3m4!1sDLsEEVDKXMmkOv61k6Xo7Q!2e0!7i16384!8i8192?coh=205409&entry=ttu

I can't readily see Castle Hall but there seems to be some buildings demolished to the left of Norwood. See this side by side Old Map/Satellite  [the cursor movements match on each side]
https://maps.nls.uk/geo/explore/side-by-side/#zoom=17.3&lat=53.27070&lon=-3.92064&layers=6&right=BingHyb
I have made a screenshot and circled Norwood/Cartref

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.

Thomas John being under 21 would not have been allowed to Vote then

Key paragraphs
The Representation of the People Act, 1918, makes special provision for the registration of members of the Military Forces. Every officer and man who is a British subject and will have attained the age of 19 years of age on 15 April 1918 is entitled to be registered as a Parliamentary elector for any constituency in which he would have been residing but for his service in the Army. A statement made by him in the “prescribed” form and verified in the “prescribed” manner that he would have had the necessary qualification for being registered but for his service, is to be treated as sufficient proof of qualification if there is no evidence to the contrary. …

From
https://www.longlongtrail.co.uk/soldiers/a-soldiers-life-1914-1918/the-registration-of-military-voters/