That is the correct type, with the bent arm. How much did you pay? I see prices from $55 to $200 for new ones. There are ones that look similar for earlier or later engines (forgot) that have a pin at the end instead of the bend and cost much less. There should be 2 gaskets w/ a steel shim between the stop valve and injection pump. You can buy the gaskets separately real cheap.

If the stop valve doesn't leak when you pull a vacuum, it is good. If your car has only ~10" Hg vacuum as both my 300D's did recently, even a good one will be very marginal in shutting off the engine, taking 10 sec or not at all. After fixing/replacing the vacuum pumps and getting a steady 20" Hg, both engines turn off immediately.

Replacing one is easy if you have a set of ratcheting box wrenches (Craftsman ~$35). I recall the 3 hex bolts have 10 mm heads. Remove the white transmission control valve on top of the injection pump first. You need to snake the stop valves behind the oil filter housing to get them in or out of place, which is easiest if you suck the rod in with your MightVac.

As mentioned, insure the hook goes around the arm in the IP arm, which you can't see. When correct, you can push the stop valve forward and won't feel it pushing anything. If you tug backwards, you will feel it catch on the arm. It isn't hard to get it right and someone would have to be inept to force it forward against the arm using the bolts. That would leave the pump at "high throttle" and I imagine one then couldn't push the manual "stop" lever full forward. If correct, you can easily slide the stop valve into the IP pump and snug the bolts up with 2 fingers, assuming you were smart enough to clean them and spray w/ WD-40.