It's not a mini-lemon, but something rather more worthwhile - a
'limequat' and probably 'Eustis', which is the most popular. This is
a hybrid between a key lime and kumquat that can be used unripe as a
lime and fully ripe as a lemon. The fruit is decidedly edible and can
be used for flavouring drinks as well as normal culinary purposes.
The thin rind is relatively sweet when the fruit is fully ripe and it
makes a very delicious marmalade. As to hardiness it is possibly more
borderline than 'Meyers', but has grown and fruited well here in S.
Devon for several years. That plant looks a tad yellow and devoid of
leaves though. A good shot of magnesium sulphate in spring followed
by regular applications of Chempak No2 (every 3 weeks throughout
summer and early autumn) will soon put it to rights. Nice thing to
have.