It's a later(ish) 50B with the plastic trigger, look after that part because they are known to fail (break) irreparably.
You will only notice the ball bearings if you take the gun apart further than in the pics, they must be there otherwise the piston would not be retained by anything.
The spring you need is nothing special, you will find that it is common with the model 35s and several similarly sized Diana/Originals so I'm pretty sure someone will stick his hand up.
Chase the vendor for the rearsight too. That is far more significant a potential issue than the lack of a simple spring.
I wonder what type that should have considering it is somewhat late, whether the typical sight for the B versions or something different. Ordinarily should be the first one in the pic http://www.maltashootingsports.com/i...rws_sights.jpg
**WANTED**: WEBLEY PATRIOT MUZZLE END; Any Diana/Original mod.50 parts, especially OPEN SIGHTS