What kind of psp have you got? If it’s a fat psp, then you can downgrade back to v1.50. but if it’s a slim you’ve got, then you can’t.
And no, you can’t use the recovery program to recover back form v3.71m33-2 to v1.50. it will brick your psp. No matter which one you’ve got.
The only way you can downgrade your psp to v1.50 (fat psp only) is to use the pandora’s battery. But get an early version of it. Because all the latest one’s will install v3.71m33-2 onto your psp.