Thanks to Discord user STLF for stumbling upon this detail! Research reveals that Union Monsters printed prior to Structure Deck: Seto Kaiba [REF] had a Condition restricting the number of Union Monsters that could be equipped to a Monster, and also the circumstances under which they could be destroyed in place of the equipped monster. Additionally, they could only be Special Summoned from the Spell / Trap Card Zone into Attack Position. This Condition would cease to be printed, and would be retroactively removed from Union Monsters through via errata. When playing Edison Format, remember that ALL Union Monsters have the following Condition:
"(A monster can only be equipped with 1 Union Monster at a time. If the equipped monster would be destroyed [as a result of battle], destroy this card instead.)"
Be sure to familiarize yourself with the specific Conditions that existed for your favourite Union Monsters at the time of SJC Edison before you start playing, just in case the current card text differs!