I did have a little retort here until I realized that the Chinese manual artworks that specific type of rocket-cart are based on are apparently undated. I thought they were from the Huolongjing which would put that specific design perfectly within the mods timeframe but until I figure out how to leaf through the thing page by page I can't arbitrate one way or another.
I will say that the design doesn't bother me much because to me the rocket compartment was innovative, not the method of mounting. It is my guess [guess, not stating any facts here] that from the moment the rocket-compartment was invented it would have been mounted on basically every kind of chassis imaginable, so I am not fretting with the specifics.
That said, if I can find the source data for the period art portraying Sly's design this debate will be pretty firmly settled on way or another, or until Sly changes his model design substantially and opens up a new question.