The SLSB has a copyright of 1988 while TR2750 has a copyright of 1989, but it wouldn't surprise me if their development overlapped. I also find it interesting that the "fusion plant" and "engine" are listed separately as though they're separate items.
Interesting. I would guess the development overlapped indeed, probably they had ideas for upgrades by then. And the ClanTech, of course.
The text could be talking about power plants generally. Like, Star League internal combustion engines were probably better than others. Alternatively, engine could refer to myomers. I mean, if we are exact, a 'Mech's fusion plant is its power plant, and myomers are the engines, powered by the fusion plant.
As for why there is no "advanced myomer" or such, the explanation is simple: The fusion engine item in 'Mech construction rules is an abstraction that, with internal structure, incorporates myomers of a 'Mech. Consider item weights in 'Mechs, specifically the internal structure. Does it incorporate myomer mass or not? If it does, does it mean endo-steel has lighter myomers too? Or are myomer weights assumed to be part of the engine? Mass being localized to single items despite fluff indicating otherwise has precedent: The Targeting Computer for example.
Royal variants are defined by the technology they have (stuff that hasn't been passed along to the other Star League member-states yet), not by how numerous the variant is.
The picture is inside the textbox, by the way, and that particular textbox is where the whole concept of "royal variants" originated. ;)
Given that there never has been any mention about a Royal (or even just Star League-era advanced) Panther, it seems likely the picture was never intended to represent an advanced Star League 'Mech, or if it was, the idea was quickly dropped (regardless of attached text).
Art in modern sourcebooks may not match the setting, content or contex otherwise, like First Succession Wars having an era-inappropriate 'Mech in one picture (either from later era or an extinct design, can't recall) and Second Succession War featuring Elemental Battle Armor for some reason on one page, these are likely due to layout and production demands. Presumably similar things may have happened during early BattleTech.
(Also, art featuring odd number of missile tubes or gun-barrels is a staple in BattleTech art. TRO2750 is especially bad about this, having many units with art and stats that do not match.)