I believe you're referring to the rebound spring and rebound spring support. If these are parts you think you might be losing again in the future, you should probably order several spares from Ruger. They're only $2 a pop and I'm sure you can think of some other things that will get you up to Ruger's $10 order minimum. Always order from SHOPRUGER.COM if you can because it's normally cheaper than the service department unless you think you can talk them into sending you something for free.
Only one of my MKIIs was purchased new, but the rebound spring and two-piece stainless bolt are the only MKII parts I've had to replace that were not clearly damaged by a user malfunction or neglect.