Tiny porringer with Leachish decoration, SJ mark?

Very broadly speaking. Generally, studio pottery that only has a written or incised  signature on tends to be by a either a talented amatuer or a professional potter who did not practice for a long time. The 75 would indicate made in the 1970's.
This was at the height of the boom in individual people learning pottery making. It was a very popular alternative life style amongst the Post war Baby Boomer generation.
        Unfortunately, it was a poorly paid job. Many excellent potters stopped working. This is an excellent professional piece. But probably unidentifiable.