I have done the adaptation of a system 1 / system 3 lockdown bar system to my El Dorado.
As mentioned, you will need the slide mechanism to go with the "shark fine" style bar.
Then, you need to pry the small flat wood spacers off the inside front of the cabinet (the spacing is now handled by the slide mech.).
Then I had to trim/cut about a 1/4" notch across the top front edge of the cabinet (closest to the player) to provide clearance for the metal lip of slide mech.
As I recall, that was all that was needed to do it.
Typically, you can get this style lock down bar for around $25 plus around $15 for the slide if you ask the seller if they still have it. The catch is the extra cost for the shipping.
FYI, There is a slight difference in the newer bars. The do seem to overhang the side rails a tiny bit more and don't hug the top edges quit as close. But that is no different than how Premier sold games for many years.