Full Idea
It may be that for Locke 'real essences' and 'internal constitution' cannot be synonymous because, according to Locke, real essences are unknowable, but internal constitutions are knowable.
Gist of Idea
Locke may distinguish real essence from internal constitution, claiming the latter is knowable
Source
report of John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.23.12) by Jan-Erik Jones - Real Essence §4.4
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.18
A Reaction
[He cites Susanna Goodin 1998; evidence for the first half is 4.6.5 and 12, and for the second is 2.23.12] One suggestion [citing 4.6.11] is that essence includes the powers, but constitution is the material components.