FW 6.38 Minimum Edition for 01g, 02g model by neur0n. From the readme:

From 5.00M33 -> OK
From 5.50GEN -> OK(may need to edit version.txt)
From 5.50Prometheus -> OK(may need to edit version.txt)
From 6.20TN-D -> OK
From 6.20PRO-B4 -> OK
From 6.35PRO-B4 -> OK
Installation:

First, you need to install CFW or HEN in your PSP.

Copy UPDATE folder at ms0:/PSP/GAME/.
Put 6.38 official update at ms0:/PSP/GAME/UPDATE/638.PBP .
Run installer from xmb.
Features:

Hold R trigger and turn on the psp, you can enter recovery menu.
Hold Home button and turn on the psp, you can Boot OFW.
Credit

This CFW is based Dark_AleX's 5.00M33. And usbdev.prx is his own.
History:

v2
Added M33driver and OEdriver.
v1
First release.

Download
Source- qj