So I've been perusing the Succession Wars book, and I really like the idea of picking a regiment descended from the SLDF. Makes it interesting in the 1SW era, as they're absorbed into the LC formations. Unfortunately...there's just not all that many of them to pick from, and fewer still that survive until the Dark Age. I went with the info from Sarna just because it's the easiest to track back and forth through, so feel free to correct me if I'm wrong.
Units I can't find any info on: 6th and 7th Arcturan Guards, 11th Hesperus Guards, Sakhalin Regulars (any of them), 181st Heavy Assault Regiment, or 9th and 18th Lyran Guards. Does anyone know of the eventual fate of these units, or did they just get left off the field manuals and aren't mentioned as getting stood down anywhere? I'm especially hopeful for the 7th Arcturan, since they're a former Royal unit.
Known destroyed: 19th Lyran Guards (Steel Viper), 5th and 14th Donegal (WoB),
Of the units that survive to the 'end' of Battletech, the 9th Lyran Guard is interesting, being a mix of several battalions of SLDF troops and (according to Sarna) a bunch of anti-Amaris insurgents from the RWR. 17th Arcturan gets to fight a lot of FWL units, including the Wolf's Dragoons back in the day, while also taking on the Clans and the Blakists for quite a while.
I wish there was more info on the 11th Royal Heavy Assault Regiment. Having a unit descended, eventually, from that force would be a fun bit of continuity; they're the only Royal unit that shows up in the LC rolls. If anyone wants to fill in some lore info that they might have on those above units, I'd love to hear it.
There's also some SLDF->merc units listed, but only a few. What happened to the 12th Heavy Assault? Most of the other merc forces are either well known (Illician Lancers) or wiped out, but it doesn't seem to have any info on the 12th's merc service after they signed with the LC in 2784.