The symbol for the base of the natural logarithm is traditionally ℯ (SCRIPT SMALL E), while the symbol for Euler–Mascheroni constant is γ (GREEK SMALL LETTER GAMMA).

The Unicode ℇ has name EULER CONSTANT. It's a bit ambiguous, but i suppose it means ℯ. However, i tried several Unicode fonts, all of them has the appearance of mirrored 3, none showed it as ℯ.