Replacing rear derailleur on Murray 15 speed - Looking for parts I have an old Murray Ultra Terrain mountain bike; it's 15 speed and I had trouble re-mounting the rear derailleur to the frame. It's broken now so I'm looking for a replacement but I'm unsure of what would fit. Here's a photo of the other (non-drive side) of the frame.



From what I understand, the number of gears on the rear cassette, denote the type of derailleur replacement--in my case, 5. Is this correct?

Where can I purchase the best replacement derailleur for this bike? I prefer to order off Amazon but will look at other options.

Here's a photo of the derailleur:


I lost the original mounting screw and had to use just whatever fit...
Likely your Murray uses something like a Falcon 5-speed unit:


Here's an image of the derailer properly mounted: