With the plenum removed, this is what the Central Sequential Fuel Injection system, (CSFI) looks like.
The image below is what each of the 6 injector/poppet assemblies look like for the CSFI system. Each poppet has two tabs that lock it into the lower intake manifold. (You can see the tabs in the image above.) The two prongs at the top of the injector is the electrical connection. All 6 injectors are in the black plastic housing in the center of the lower intake manifold. When an injector is fired, it squirts fuel on the back side of the intake valve. The injectors are individually fired in sequence with the firing order.
This is the Multiport Fuel Injection, (MFI) upgrade. The poppets have been eliminated and the injectors have been relocated to where the poppets used to be. After the upgrade, it's actually Sequential Multiport Fuel Injection, (SMFI). This upgrade essentially changes the 8th character of the VIN from "W" to "X".
On 95 and older 4.3L "W" engines, only one injector with 6 poppets is used. The injector is "batch fired" which means it delivers fuel to each cylinder any time any cylinder is calling for fuel. This system is called Central Multiport Fuel Injection, (CMFI) and is less fuel efficient, that's one of the reasons why the CSFI system replaced it.