Speed gauge question?

[pursuit2550]

What is the black plug and the black box on the back of this gauge. I have never seen this on any gauges.
YAMAHA BOAT DIGITAL SPEEDOMETER GAUGE 6Y5-2818R-30:eBay Motors (item 200388207642 end time Oct-03-09 10:08:33 PDT)

[boats.net]

Thats an old gauge. The square box is a diaphram(needs to be removed, causes incorrect speed) and other harness is to connect to fuel management gauge(old style).