IIRC-that is when they change the early lifter from a non-oil through lifter/solid PRs to a oil through lifter/solid PRs and later on to a oil-through lifter hollow 5/16" PRs in the later 90-91 pre-magnum LA roller cam engines as they gear up for the 92-93 magnum engine debuted. you have to WATCH for non oil through lifters going into a magnum engine that oil rockers via PRs

I have also found in a lot of the 90-91 pre-magnum LA roller 5.2 engines that they oiled through the heads/shafts as well as up through the lifters/PRs. IMHO this is why a lot of them had VC leaks as it is a lot of oil in the VC area at operating speeds & that the PCV valve suck a lot of oil as well. (why I use a cheap oil/air separator on mine) on the police engines they have grooved #2 & #4 cam bearings for full time oiling to rockers and they tend to have a lot of VC leaks also.

I install restrictors in the block reducing the feed to rocker to 1/2 the oem block size. along with all the oiling mods I do.

all of my lifters are the same part # & yr, they are sealed NOS-1989 OEM replacements- oil through lifter/solid 5/16" PRs. I index them when installed hole facing cam per FSM book.

installed dry/not pumped up and all PRs where at zero lash/no preload per cyls. = I could pre load bottom out each lifter by hand with no loose rockers. then primed oil pump/engine - Lucus sae30 break in oil with 65 psi with a 1/2" drill. they all pump up and primed till no air bleeding out of lifters. rotating engine to oil both sides all 16 rocker oiling.

all on a HughesEngines roller cam 192*/203* .500/.513 lift 114* LSA

I should be good to go.