In FileZ and ZLauncher Palm OS reports itself as version 5.0 development build 0. I'm not sure if this is just limited to my device (it had a "Refurbished" sticker on the back) or if it's like that.
FileZ says "Palm OS v. 5.00d" by the way.
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Palm III (RIP) -> IBM WorkPad 30X (RIP) -> Treo 90 (RIP) -> Clie NR70V (Retired) -> Clie NX73V
Go to the PalmOS standard Launcher, tap the menu and select the option called Info... It shows you a popup window, select "Version" on the bottom and it will report you the PalmOS version of your device. What version shows up?