I have been working with Galil products for over 20 years and this is the first time I have heard of a D300 breakout, nor can I find any information on it. It may have been a special device built specifically for an OEM. You might check with Galil, they may be able to provide one, but they may direct you to the OEM. Galil Motion Control
This is what I use for a breakouts, the listing is about 1/2 retail price. https://www.ebay.com/itm/PHOENIX-CON...IAAOSwihxc9Y8M
But the above breakout may require a relay interface to use the 5V outputs. The ICM-1900-OPTO might be a direct replacement. In either case you would need a 100 pin cable.
This might be worth a look. https://www.ebay.com/itm/Galil-DMC-1...EAAOSwUuNctn40
I don't think the above has the -OPTO option, but not sure. But you get the cable and a DMC-1840 as a bonus, pretty good deal.