How to determine what buffer was changed externally with gVim?

From :help FileChangedShell:

NOTE: When this autocommand is executed, the
current buffer "%" may be different from the
buffer that was changed "<afile>".

You need to locate the window where the corresponding file is being edited. For that, the buffer number (in <abuf>) is even easier:

let winNr = bufwinnr(0 + expand('<abuf>'))
execute winNr . 'wincmd w'
edit

The same applies to the buffer name; replace

let MyBn  = bufname("%")

with

let MyBn  = expand('<afile>')

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