There appears some confusion about who was, or what counts as, the first pastor of Jerusalem Chapel. Ystalyfera. The South Wales Voice of 7th July 1951 states:-

THE FIRST PASTOR
For many years however, they could not afford the services of a pastor; the first to Officiate in that capacity was the Rev Ivor Jenkins, who retired in 1913

Conversely, the South Wales Voice for 13th December 1957 states:-

The first Minister was the late Rev John Jeffries who took office in 1884 and was succeeded by the Rev J Ivor Jenkins in 1902 and he in turn was followed in 1914 by the Rev D W Stephens.

Nevertheless, thanks to the newspaper, we have a full listing up to 1957:-

Rev John Jeffries was pastor of Jerusalem Chapel from 1884 until 1902.		Rev Ivor Jenkins was pastor of Jerusalem Chapel from 1902, and retired in 1913.
Rev D W Stephens was pastor of Jerusalem Chapel from 1914 to 1926.		Rev D R Beynon was pastor of Jerusalem Chapel from 1928 until 1953.
Rev D T Davies was pastor of Jerusalem Chapel from 1954 until 1957.		Rev Hugh Llewelyn Jones was pastor of Jerusalem Chapel from July 1957 onwards.