The original Yuan Yang Zhen differed from later derivations on several aspects. It consisted of only one five-man team (half the size of later version), and did not employ rattan shield, Tang Pa (钂鈀) or quarterstaff. The five-man team consisted of one shieldman, wielding nothing but an Ai Pai (挨牌), who was also the team leader. The team leader stood at the front rank, with his shield angled slightly to the left. He was followed by a soldier wielding Lang Xian (狼筅), with his weapon pointed slightly to the right. The rest of the team consisted of three pikemen armed with a long spear. Additionally, the team could be supported by more archers, handgonners and arquebusiers.
On flat terrain, large number of Mi Zhan teams could be deployed together into one big block (or several big blocks). Shieldmen formed into shield wall to protect Lang Xian and pikemen against attack and missile fire, while Lang Xian functioned as mobile wire obstacle to disrupt enemy attack and prevent enemy troops from getting too close. This formation effectively deprived its enemy of all means of retaliation, be it spears, pikes, polearms, short weapons or missile weapons. However, it was vulnerable to gunpowder weaponry that could punch through Ai Pai easily.
Comparison with later versions
Compared to later versions, this prototype formation was a lot more simple, and geared towards large scale, pitched battle. It lacked the flexibility and offensive capabilities of later Mandarin Duck Formations, but enjoyed a lot more staying power (later Mandarin Duck Formation used alternating rattan shield and Ai Pai, so it couldn't form shield wall as effectively).
Other blog posts in my Mandarin Duck Formation series:
Mi Zhan — the original Yuan Yang Zhen
Qi Ji Guang's Yuan Yang Zhen — part 1
Qi Ji Guang's Yuan Yang Zhen — part 2
Qi Ji Guang's Yuan Yang Zhen — part 3
Qi Ji Guang's Yuan Yang Zhen — part 4
Qi Ji Guang's Yuan Yang Zhen — part 5
Qi Ji Guang's Yuan Yang Zhen — part 6
Xu Guang Qi's Yuan Yang Wu