I suggest you ignore the WW1 soldier I found; I think he's red herring.
I assume your man is Samuel Issac Boulton born 3 August 1871 (according to the 1939 Register) who married Susannah Brooks Vinnicombe on 30 Mar 1903. The census for 1911 confirms 1871 as his year of birth, as I expect does the 1921 census although I haven't checked it (£).
This is slightly at odds with the militia attestation which Shaun found, dated 15 January 1894, where his age was stated to be 21 years and 5 months, resulting in a birth date in August 1872, not 1871. I don't think we should read too much into this discrepancy as all the other factors support this being your Samuel.
So on that assumption, if he joined the South Wales Borderers Militia battalion then he will have done his basic training at the SWB depot at Brecon, alongside the Regular recruits. And since he joined the Regulars 1½ months later on 1st March 1894 I think it’s fair too assume he was strongly encouraged to do this during his time in the depot, and therefore he will have joined either the 1st or 2nd battalion SWB.
At the time the 1st battalion was in Egypt and they moved from there to Gibraltar in 1895 and to India in 1897. They remained in India until 1910. The 2nd Battalion was in Aden in early 1894, but returned to Portsmouth in November of that year. In September 1895 they moved to Aldershot where they spent two years, followed by two years in Pembroke Dock, and then to Dublin in 1899. Their Ireland tour of duty was cut short in early 1900 when they went to South Africa.
If you are able to get to the National Archives at Kew, you can resolve the issue of which battalion he joined by checking the muster rolls for 1894, which are in
WO 16/2997. Unfortunately these rolls haven’t been digitised.
Incidentally if, as I strongly suspect, he did join either the 1st or 2nd battalion SWB, his new regimental number is likely to have been around 4130-4149, as number 4150 was issued to a soldier who joined on 13 March 1894.