Manufacturing techniques have changed a lot in the past 30? years. That cable assembly is complicated, and build quality is high.
The ring-connector is for being screwed in to the keyboard chassis (which is metal, underneath). This will ensure that the keyboard ground is at the same potential as the actual computer. It will also discharge any static that builds up on the user, keys, etc. Having a ground-strap is not surprising, but what does surprise me is that the grounding is separated from the main cable bundle where it is.
I suspect that during manufacture, the 6# 'tail' of the ground cable is separated out from the other cables, and terminated with the ring connector. Then the other conductors (data signals) are terminated in the connector-block. The cable is then re-molded and insulated with the strain-relief block in a separate molding process.
The Strain Relief is visible on the outside of the keyboard, and the molded block sits inside the keyboard, creating the mechanical mount for the strain relief. The block probably contours well with the inside of the keyboard, and prevents 'wiggling'. It also protects the junction where the earth conductor re-joins the rest of the bundle.
Reasons for building cables like this are not normally for cost. The 6" tail is probably needed for regulation purposes... You can bet that at the time the keyboard was built, there was a requirement that "all devices need to be grounded within 6 inches of where any power cable enters the device" or something. Using a complicated system like this is probably cheaper than having the cable routed differently to get the data to the circuit-board.