The kit is in fact a service-installed upgrade of the machine. The upgrade consists of a set of LED lights on the printer platen and a light sensor on the carriage – these are used to track and align the image on each side. The kit also includes output zone lighting (like the HP Latex 3000 Series Printer) —frontlit and backlit — so that registration can be checked by the operator.
HP Latex 1500 Printer
Double-sided Day and Night Kit Part Number P4P91A
HP Latex 3000 Series Printer
Double-sided Day and Night Kit Part Number J8J14A
If you want learn more about it please read the following article Double-sided Day Night Kit