Toshiba Satellite Pro 430CDT - POST code monitor on printer port
This information may be suitable for the Toshiba Satellite Pro 430CDS and 435CDS as well.
Starting on page 2-10 of the 430's maintenance manual is a procedure ("Procedure 2") for using a 'Printer port LED' (a Toshiba device) to monitor POST codes send to the 430's printer/parallel port.
A modern equivalent of the 'Printer port LED' is shown at here.
It can be purchased from eBay, usually appearing in the results of an eBay search using: laptop post tester
The device has two 7-segment displays, not the expected one display (only one is required to display a byte), and so the reason for two is probably the same as that for the POST card shown at here.