Speculation only but I think they hardened the lead alloy in some and others increased jacket thickness and some they did both. My guess is either the 154 was just enough slower it was Ok or it is not as popular as the 139 so has not gotten the treatment. Some I have sectioned had barely perceptible interlok rings. I thought it might be because of the way they were sectioned but didn't melt any to really check them out. It would be nice to know which ones were re-vamped and which are not as now I currently avoid all of them due to my initial experiences with them.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli