Stonewall
Chances of finding original "Manhattan" lock work parts for your example will be akin to a search for the Holy Grail.
Historically, as with several other manufacturers, when the Colt Patent(s) expired, others jumped at the chance to copy Colt Pattern guns. Manhattan was one of those.
What you describe is typical of having a functional Main Spring. the gun may or may not have a functioning Trigger/Bolt spring. Pull the trigger - does it rebound?? It also sounds like a broken Hand Spring or even a broken Hand as well. A broken hand spring is a simple fix, by replacing the broken spring with a Uberti spring. It is also possible the hand is the same as a Uberti hand, which copy the OEM Colt items. These parts will NOT be drop in and will require a degree of fitting. Should be able to return your Manhattan to functionality. It may well even be shootable with lighter loads. Best of Luck with your endeavor.
Type 0 corrected. I detest OTTO