You can't, there's no dates marked on TM guns. You might have a manufacture or QC stamp on the gearbox. It's not important to know when it was made. TM Mp5s haven't really changed for a decade.
And yes, these things are expensive, but airsoft IS expensive. These parts are unique and aren't in demand that much to be mass manufactured to the point where they are really cheap. That $62 (probably $70 shipped) silencer is not so bad. If you were to buy the thread adapter it would probably cost you $30 at least and then you'd need to buy a silencer as well for at least $40. That's $70 right there. If you buy the QD one, you don't need a thread adapter.
The $36 one is better than the $20 one (probably $30 shipped since it's Canada Post) because when you take the silencer off, you still can use it as a regular flashider if you screw on the cap. The $20 one leaves you with no flashider at all as all it does is put threads on the end of the gun.