About when did Lionel introduce the riveted core in the ZW transformer? Some refer to it as a “solid core”.
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The ZW type R core redesign was done around 1957.
Note that the 275W rating was instituted in 1950 while the laminated core was still in use.
The ZW core redesign was done some time prior to the final 1957 revisions that resulted in the "R" model with the 18 volt pilot lamp. It was a running revision, all "R" models have a riveted core, but a riveted core does not make it a "R" version.
At the risk of being a pedant, 
ALL transformer cores are laminated ( a stack of thin steel pieces with each individually varnished to electrically insulate them one from the next). This is to prevent eddy currents in the steel core (such eddy currents cause heating of the core and thus a decrease in efficiency). The stack of laminations is held together either by riveting or by welds along the edge. Welding is the more modern technique.
What Lew stayed above is true except that no Lionel cores were welded. My opinion is that all changes over the production life of the ZW were to lower the cost and labor time to build them. There are several versions of ZW type/Model R with the newest version having less parts to assemble and solder. Are the last R versions better than the first ZW 250 watt? Maybe a little bit. All versions supply only 180 watts total Power to the track.
@Tranz4mr has a webpage that goes over some of the ZW variations: http://www.tranz4mr.com/ZW_VW_Variations.html
