I'm about 90% sure that the guise was the common drop from the Assassin on live. Of all the nameds in Lguk that have Armor/Weapon drops the weapon is the rare and the armor is the common. The exceptions to this are the Savant and the Crusader. The Savant's EBM and Crusader's RF are the commons because they don't have any additional effects or stats. We do know for sure that the Mask of Deception is common after the Guise was removed which would have required them to shift the rarity of the SBD which seems unlikely as all the other proc weapons are rare. The guise prior to its removal was not really that special. The renown that is has was gained after the drop the nerfed.
Why does this matter? When P99 went live the guise was set to rare. When the guise was replaced with mask the mask was also kept at rare for awhile until I bug reported it and it was fixed. In order for green to be a truly classic experience and keep Lguk from being a complete shit show I suggest that the rarity of the guise be reevaluated to the common drop unless evidence claims otherwise. I've looked over the years for posts that talked about its rarity and never really found anything. However, the in game loot for lower guk would suggest that the SBD belongs to the rare cycle of proc weapons and the guise is part of the common cycle of equipment.
edit: Actually, here is an EQ'Lizer page citing that the guise was the common drop. Apparently I just didn't look hard enough before.
https://web.archive.org/web/20000410.../lowerguk.html