You are right,
the numbers only specify the language.
(and version 1.4 is german, just to have this complete)
No significant / if any updates are included in the higher versions, beside some localization code. (See
here if you are interested in)
It is proof of file, that you lose no functionality if you install the 1.1.00.1840 US version.
prs-customizer is no "official firmware", it is a set of hacks and patches,
e.g. Porkupan's localization javascripts, which will be integrated into the original scripts to add functionality.
In the end providing a preconfigured Univeral-Flasher to "flash" the changed/modified files to your PRS-505.