As promised, Dark_AleX has released the 1.50 kernel add-on for Custom Firmware 3.71 M33. The add-on works only for the older PSP models (PSP-1000), and installation instructions are included in the zip package. The add-on allows you to run homebrew that runs on the 1.50 kernel, which was not included in 3.71 M33.
“The program will reject PSP Slim, due to the 1.50 kernel not working on it.” So, this is good news for those who are on the older PSP model who are craving some 1.50 homebrew.