Well I stink at looking at titles, I guess. I run an SM318, not a 420. But I didn't see that before I went and tore mine apart so here's what that looks like. Surely yours is similar. Heck, this style was used up to the 80's or so in one form or another!
This is probably way more detail than you need, but if someone else needs it at least it's posted. Here's a look up inside the hole where the speedo cable goes. Notice that it's square so it can hold the cable and turn it.
You take off the hold-down bolt on the side and that releases the clip holding the cylinder in.
Once the clip is out of the way the cylinder just pulls out. You might have to twist it back and forth a little if it's rusted or painted on.
This is the cylinder all the way out. That yellowish goopy looking thing sticking out of the cylinder is the plastic piece that busts up sometimes.
That plastic piece just pulls out. It's not screwed in or anything. Here's what it looks like.
And that's it. If yours doesn't have a plastic thingy at all it has probably been tossed by someone that didn't want to put miles on their truck.No plastic, no odometer, truck never drops in value...It was common in the older days. Or it just disintegrated one day and nobody bothered to replace it.