The only files that are missing from this version are the downdater files which you must obtain from a firmware 1.5 user. Here's what they need to do:
1. Copy the folders downhelper and downhelper% from the folder 1.50 HELPER folder to /PSP/GAME/ in your memstick.
2. Obtain the 1.50 update EBOOT and put it in /PSP/GAME/UPDATE in your memstick. You may find the 1.50 update in our download site here.
3. Init the downhelper program.
4. It will dump your flash and some files from the updater eboot to the memstick. (These files are illegal under US copyright laws, so please do not distribute them in the comments or on our forums)
5. After that, the program will exit. You can now delete the updater from /PSP/GAME/UPDATE
6. You'll notice that you have a new folder in the root of your memstick called "DOWNDATER".
7. That's the folder you'll have to send to a 2.50/2.60 to let them test the downgrade.