Port Talbot came under the Neath registration district.
Just 2 Trevor Williams about the right time:
Trevor S Williams, March qtr 1911
Trevor Williams, December qtr 1913 - mother's maiden name Thomas.
There are 6 Reginald Williams registered 1915-1917:
June qtr 1915, Reginald - mother's maiden name Stewart
March qtr 1816, Reginald A - mmn Stephenson
September qtr 1916, Reginald W E - mmn Bufrey
June qtr 1917, Reginald L - mmn Williams
September qtr 1917, Reginald T - mmn Roberts
December qtr 1917, Reginald N - mmn Jones