IMO, (been there) you should do nothing but consult a sxs smith. Charlie @ Hill Rod & Gun is DA MAN in the US high plains.
With all due respect to Charlie, he is a gun retailer, not a gunsmith.
Your mainspring is broken. As you probably know already, the mainspring in the Lancaster Model A is also the cocking limb and the power for the self opening mechanism. It's a very big, strong spring. Pretty rare that they should break, but like anything else...they can do just that.
Give a guy like Dewey Vicknair, Abe Chaber, Kirk Merrington or Stephen Hutton a call. They can make a new one and have the gun shooting for the next hundred or so years.