Ok, you have installed the so called "HardSPL", which is a custom SPL which unfortunately doesn't allow you to flash recovery / radio / SPL using fastboot.
Now you have different possibilities:
1) keep your spl / radio and flash a rom which supports your SPL/radio. The old enomther roms, CM4/5 should work without problems
2) keep your spl / radio in combination with custom partition tables. In that case, you should be able to run CM6 or other Eclair/Froyo based roms, too.
3) Update your SPL and/or radio and you will be able to run all roms available for the G1
In your case, updating the SPL is to be done in recovery mode. This is a bit more risky that doing so in fastboot, because flashing is done while setting the "reboot into recovery" flag. If for some reason the recovery cannot be started correctly you will have a brick.
There are some tutorials here in the G1 area guiding you through these steps, unfortunately I don't have the links available at the moment, but if you search for it, you should find it easily.
Btw. if you really want to return to tock, you can also flash an original image ...
1. Phone: Samsung Galaxy SII GT-I9100, CM-11-UNOFFICIAL-i9100 (ADS-build)
2. Phone: G1
Rom: IcedADS_Magpie Gingerbread (2.3.7) based on ezGingerbread and Firerat's magpie
Nexus 7 (16GB), Android 4.4.4 (Rooted Stock)
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