wild plum? i.d

came across these today, the yellow ones were very ripe and ready to eat, stunning sugery sweet flavour, as you can see by the pictures very juicy fruit, that seperated from the stone easily
the red type still a bit hard, (look and feel more like cherries)
so what could they be?
and how do YOU, go about trying to classify plum types fruits,

my guesstimate was they were either cherry plums /myrobalans or mirabelle,

not being a plum expert i thought myrobalans and mirabelles were similar or one and the same but have found out to the contray they are not the same,

were they grow there is many weird and wonderfull plum type fruits some which dont ripen untill november-december ( which i think maybe some green/yellow type bullace
will get pics when they are ripe) i have been sent pictures by someone in the darbyshire area calling them winter crux or crax, ( think this maybe a locale name like some call winberries orther call billberries, or bleaberries)

also one that ripens early september that i can only describe as a yellow/ orange damson!!! again i will collect and take pictures once ripe fruit is collected

love wild fruit,