Mk IV Rocker Shaft and Posts

Further to my earlier posts on the pushrod springs issue, I have discovered more variability in the parts in my collection and some issues which might be of interest to others.
Firstly when I dismantled my rocker gear I noticed that the rockers slipped easily off the shaft. Then it was apparent that there was significant wear on the shaft, but not at the 12 points where the bushed rockers worked against the shaft, but where the shaft was held by 5 of the 6 posts. My measuring techniques are not up to much but I can see and feel grooves which on one side (top) of the shaft appear to be worn down about 2 thou. ie the shaft appears to have been moving up and down in the posts. The (first?) post with the oil gallery is fixed with the hex headed screw and not worn but the others are.
Secondly, on examining my spare sets of rocker gear, I find there are three types of posts in existence. I am not sure of the origin of these parts but assumed they are MkIV but have been in possession of some SS parts in the past also so there could be a mix up. Anyway, the posts from my (original) engine are solid, with no clamping action in the ring that carries the shaft. I also have a complete set of rocker posts which are cut through all the way on the stem which enables the shaft to be clamped securely in the post. Another spare and complete rocker set and shaft (which I will be using to replace my worn originals) has the stems of the posts cut part way only, also enabling the shaft to be clamped.
Back to the pushrod springs story, and my engine status. I discovered alloy con- rods when I opened the sump. I understood that my engine number (S3354) signified a later engine in the MkIV series consistent with the factory dating certificate of August 1948 so where and when were the steel rods fitted to MkIV?
So why is it that my engine (60K miles) has wear on the shaft which would not be seen if it had posts which enable the shaft to be firmly clamped - something that might have been rectified with split posts? Perhaps they were all like that on the ‘S’ engines and I have posts from MkV or ‘Z’ engines in my collection?
Any suggestions?
Photos attached show shaft wear and the three types of posts.
Cheers Darryl

Some of the shaft pillars have slots cut in them such that tightening them onto the head also clamps the shaft. I guess these are the later type but I haven’t checked. Earlier ones don’t have the slot. The MkIV oil feed passes through the head and up through the rear pillar. The SS oil feed has a separate pipe from the head with a banjo screwed onto the end of the rocker shaft so no passage in the rear pillar.

Engine number M1 Steel rods replace alloy
Engine number S151 Alloy replace steel rods