from the BMD index accessed through a FHC today:
there are four John Jones fitting description of my ggrandpa ie born Apr-June 1863 in Tregaron.
All are in Vol IIB. At least two are on page 89
and there is also one on page 85 and one of page 87 of Volume IIB.
What is the most economical next step to see if one of these John's has a father named David who was born in Tregaron and mother Mary?