That 'interface' is the original 'bodge' to fit an XLS to the Allegra.. It did not work well!
Massoth now do a 'special' XLS to fit the Allegra. - I have not looked, but there is probably a downloadable document on the Massoth site about it?? It is a different PCB with the connectors already on it.
If you mean a 'old' interface, is it not the six pin?
If you mean the ten pin, go by the colours, NOT any 'lumps and bumps' on the connector for polarity and alignment.. I have ten pin cables with the connectors either way round.
From experience with the Allegra, if the Marklin decoder(s) are as cheap, it is hardly worth the bother.. - The Marklin (full function and sound) decoder for the Allegra is £65 - £70 for example.