I'm afraid zeno is inaccurate here. I think I have the Spitfire II pilot notes without any amendments, at least the amendment list is there and empty. This should be the original version dated July 1940. In there, the engine limitation for all out level is 9lb boost.
It appears that the all out level increase was at latest added with
amendment list 31 / L, at a date that is not known to me.
The zeno version is missing the amendment list and does not shed any light on the "when".