yugioh ancient gear structure deck may be 1st or Unlimited Edition Amazon.com: Yu-Gi-Oh! - Ancient Gear

Targeting your own creature helps save it from a removal spell, or can give you double the triggers when it enters the battlefield

Mythical beasts, such as Master Cerberus or Jackal King, are often included because they leverage spell counters, and extra spellbook cards, including Spellbook Magician of Prophecy, will increase your draw rate, which is essential for playing a Pendulum deck

Market Price $0.70 Low $0.48 Median $0.76 Prices from TCGplayer, updated 8 hours ago

Hopefully, that deck wants four copies, though its possible they just dont have all the slots available

It is horrifically vulnerable in that time to tap effects, bounce effects and exile effects which Progenitus isn't