1451 is the date the hwacha is supposed to have been invented in
Korea. Was there a precursor with a similar configuration in the
Ming dynasty prior to 1451 that is not like the earlier huoche
version? I have no idea regarding the construction of the huoche
used Jingnan war (1401-1403). But jugding by the "Wubei zhi"
image we can assume that Mao Yuan Yi just copied this pic from
Wujing Zongyao (ashe used samples from more than 2000 old
treatises). IMHO the answer is in original edition of Wujing
Zongyao. Let me quote the article from Korea (by Yang Sung-jin)
regarding hwacha. On Jan. 16, 1451, King Munjong appeared in a
military exercise taking place near the palace. A total of 700
soldiers formed a tactical lineup composed of a mock attack and
defense. The operation was orderly and fine-tuned, the Annals
historiographers observed. Meanwhile, cannons were test-fired in
front of the king. Then, the soldiers performed a demonstration
of hwacha (fire cartwheel) which shot through all the targets
positioned 80 st