iPhone 3G thinks it's on 3.1.3 BUT redsn0w DISAGREES!!!!

[messedupiphone]

so I managed to downgrade from 4.2.1 to 3.1.3 with iTunes, got error 1015, fixed it with iREB, all up and running. iTunes and iPhone both display fw 3.1.3 - SO FAR SO GOOD.

BUT when I tried installing Cydia via redsn0w (as I did on 4.2.1), upon navigating to the same 3.1.3 fw I used for the restoration (official fw from apple) it tells me "Unable to recognize specified IPSW"

However, when I navigate to 4.2.1 it says "IPSW successfully identified"!!!!! With sn0wbreeze it's the same...

This is DESPITE the fact that both iTunes and the iPhone display 3.1.3 as fw... I have tried to proceed having selected 4.2.1 and it almost completed - redsn0w said it had finished but the iPhone Screen went black with only a white spot in the upper left corner -> I had to restore it again.

Now it's on 3.1.3 again (according to iTunes/iPhone...) and I wonder how I can get Cydia on it??? On 4.2.1 redsn0w (0.9.6b6) worked perfectly well but since many Cydia apps don't support iOS 4 I downgraded. With sn0wbreeze it's the same.

After the first downgrade to 3.1.3 I got tinyumbrella which was able to save the following SHSH:

INVALID X.X.X (XXXX)
iPhone3G 4.0 (8A293)
iPhone3G 4.0.1 (8A306)
iPhone3G 4.0.2 (8A400)
iPhone3G 4.1 (8B117)

Device Model is MB497DN/A
Installed Baseband Version is 05.15.04-5.9_M3S2

Maybe the problem is that there isn't an SHSH for anything below iOS 4 on the phone anymore? In that case I'd still find it extremely strange for iTunes to be able to restore it to 3.1.3 (with a little finishing up help from iREB) yet the jailbreaking tools don't even recognize the fw??

Can anybody make sense of this or tell me how I could possibly install cydia now???

[i.Annie]

You would use the 3.1.2 ipsw even though you're on 3.1.3 Redsn0w doesn't recognize 3.1.3 ipsw so it uses the 3.1.2 version instead.

Oh and you're not using the right version of redsn0w. It recognizes 4.2.1 bc you're using the latest version. 3.1.3 and 3.1.2 use redsn0w 0.9.4