There's a guide here on the site about that. You can supposedly tell what firmware a PSP is by a certain letter on the box. Hold on i'll try to find it but I fear 1.5 is going to be hard to find because it's an early version. You're better off getting a used PSP or trying to get one below 3.10 and downgrade it. If it 3.03 you'll need an old copy of GTALCS to downgrade it because that's where the exploit is.
Doubleposted Message Below:
Here ya go:
http://psp3d.com/getting-started-hel...tml#post182917

Read post number ten

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