iPhone 3GS "device is not eligible" error while trying to Restore below iOS 4

As Usual Apple closes it's signature verification for iPhone 3GS for Restoring below 4.0 because it will always close old firmware verification whenever new version will come.

So People having problem has only one method to use,that is Custom 4.0 at the moment (old bootrom only)

If you try to use Custom firmware on new bootrom you will get error 16xx,3194.

 Various methods to check Bootrom version of 3GS

1. Model Number of New Bootrom phones start with "MC"
2. Serial number is XXX4XXXXXXX or XXX0XXXXXXX
3. Handset should be in DFU mode
Device Manager on your Windows PC.
Look for 'Universal Serial Bus (USB) Controller', you will find 'Apple Mobile Device USB Driver' as a sub item there.
Right Click and choose 'Properties', navigate to 'Details' tab and select Device Instance Path from the dropdown list.
At the end of this information string you will find the bootrom version, i.e. (iBoot-359.3.2) or up who knows .