Go to Settings/System Settings/ System Information
The PSP will display something like this:


Thats my firmware version (3.80 M33-2), a custom firmware version (or sometimes they called it homebrew version) made by Dark Alex. Dark-AleX - PSP Developer

What I mean by M33+ are versions with M33, M33-1, M33-2, M33-3 .... and so on

In this post, this plugin to charge PSP via USB cable will surely run on
Custom firmwares 3.71 M33, 3.71 M33-1, 3.71 M33-2 and 3.71 M33-3
and will never run on
Custom firmwares 3.80 M33, 3.80 M33-1, 3.80 M33-2, 3.80 M33-3, 3.80 M33-4, including Non-M33 versions and all original PSP versions released by Sony)

Check the M33 version from wikipedia --> PlayStation Portable homebrew - Wikipedia, the free encyclopedia

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......... cabanerofr .........