thank you osxlatitude gurus. I just completed installation on an old D620 perfectly using your instructions.
I did not have access to an Apple machine.. nor Linux. Hence had to do everything from scratch from an existing Windows machine.. including a Virtual Box snow leopard installation first so as to be able to create the USB stick.
I installed from original media snowleopard106.dmg
Build_10A432 with MD5
$817F2D9CFB56885D6E8E3FD99855B691
1d1fb7eca55470143c153f7f902b4ada
I had to convert the DMG to an ISO using dmg2img-1.6.2-win32.zip
Once that was done, i installed Virtual Box 3.2.8 and Snow Leopard.
Once Snow Leopard was installed and running in VirtualBox, I used Disk Utility running within the SL virtual machine to create the USB stick from the original media. Then I installed the bootloader on to the USB and the Extra.zip etc. Once that was done, I was able to boot the USB from the D620 and complete the install.
thanks again.