18 cards where the text includes “prototype” and the set is “bro”

Arcane Proxy (The Brothers' War #75)
Autonomous Assembler (The Brothers' War #34)
Blitz Automaton (The Brothers' War #158)
Boulderbranch Golem (The Brothers' War #197)
Combat Thresher (The Brothers' War #35)
Cradle Clearcutter (The Brothers' War #198)
Depth Charge Colossus (The Brothers' War #78)
Fallaji Dragon Engine (The Brothers' War #159)
Goring Warplow (The Brothers' War #120)
Hulking Metamorph (The Brothers' War #79)
Iron-Craw Crusher (The Brothers' War #200)
Phyrexian Fleshgorger (The Brothers' War #121)
Rootwire Amalgam (The Brothers' War #203)
Rust Goliath (The Brothers' War #204)
Skitterbeam Battalion (The Brothers' War #165)
Spotter Thopter (The Brothers' War #80)
Steel Seraph (The Brothers' War #38)
Woodcaller Automaton (The Brothers' War #292)

This search is finished. Now the real work can begin.