I saw the gravestone picture in a published tree with those names and the baptism of Margaret Avarina.
That tree has John Methusalem Jones b July 1853, Crickhowell which seems wrong.
The death date is given as 29th October 1854, Merthyr Tydfil.
There is a death for Methusalem Jones in M T which would fit.
The gravestone looks like October (not sure re year) and 1 month

It’s hard, isn’t it. Maybe they didn’t register all their children.