Can you give a few details? where/how was it measured, was the value .511MeV found right away, etc..
Readers who'd like a proper answer to the question may find this link useful.
The value found by this experiment is .537 MeV
I do hope that someone can answer the question from an historical point of view, specifying when this experiment was made first, and if Einstein's conjecture (we all know that his 1905 paper was not even intended as a proof by himself) was verified by any different experiment.