It is actually for the 505, but he was trying to jump start a fix for the 300.. (from igork's post 5).
This tool should let you get the reader out of the firmware update mode. Just connect the reader to PC and run it. Note that this will put a PRS-505 firmware on it which is not really compatible with 300, so you will need to reflash it with a PRS-300 modified firmware again.
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