Ok, the decompression lever actuated the decompressor on the left side exhaust valve. It did this by turning a shaft that opens the exhaust valve a bit. At this same time it closed the start switch, which cranked the electric starter.
This was on the stock set up for the first two years of the e-start. They upgraded the starter and used a stronger battery in 74. So the decompressor was no longer used. They removed the whole mechanism and replaced it with a cover. On the throttle housing the removed the lever and reconfiguered the starter switch into a pushbutton.
On your throttle housing, if you are using the stock BS38 carbs then you need a dual pull throttle. From 70-75 they used two separate carbs, each with their own cable.
In 76 they went with linked carbs that only used one cable.
It's hard to tell in your pic just what carbs you have besides being BS38's.
Leo