Hi Kenneth

Attached is a line drawing of the bronze replica crossbow excavated near the Qin terracotta warrior pits...
From page 109 of the Chinese language book "The Bronze Chariots and Horses unearthed from the Qin Shi
Huang Mausoleum -- An Excavation Report", published by Cultural Relics Publishing House (Wenwu
chubanshe), Beijing 1998.

As can be seen from this line drawing, there is a bronze housing which contains the crossbow
mechanism. So although the actual crossbow mechanisms unearthed (together with other bronze weapons)
from the terracotta pits were not found with any form of housing, it does not mean that this applies to all
Qin crossbows... I think I will contact Stephen Selby about this...

Talking about ergonomics, notice the curvature of the stock near towards the bow, presumbably for the
non-trigger hand to grip comfortably...

Another confirmation -- An excavated Qin crossbow mechanism with an inscription dated Qin Dynasty 224 BC, excavated from the early Han "Mawangdui" tomb at Changsha, Hunan province... page 297 of "zhong guo wen wu jing hua da quan" by Taiwan Commercial Press, Taipei 1993..

In fact, I have a reproduction Han crossbow and my observations do not match your suggestion :

1)-the bowyer that made it for me told me to place my left hand very far ahead near the bow but it is very unconfortable to do so, spontaneously my left hands places itself at about half stock length.

2)-even without any curvature on my stock, which is quite thick, the string still manages to often catch my fingers or hand. I would say that with such a curvature, you'd be hit every time.

See on Stephen SELBY's pictures below that the left hand of the shooter is supposed to be placed in the hole mid stock :

...