the keys to the counterfeit coins seem to be:
1- yellow metal central disk has strange coloration as compared to normal coins. I.E. darker, more of a brass color, and sometimes showing coppery colored staining in areas, especially near the edges where it contacts the white metal outer ring.
2- Edge lettering/design is either non existant (sure fire giveaway of a false coin) or weak or very dissimilar than on the normal coins.
3- design of the coin suffers from very grainy appearance, and will be noticeably very weak when compared with a real coin. almost like a 'sandblasted' appearance or as if the coin was struck through grease. This comes from the counterfiet dies, as apparently real coins are used to make moulds to cast the dies with
4- weight. False coins will differ markedly
5- magnetism. Areas of the False coin will take on much different (stronger/weaker) magnetic aspects of real coins. this is also a 100€ indicator.
Some coins appear also to be struck entirely in brass, or other yellow base metal and then have the outer ring electroplated.
Your coin pictured appears to be a normal 2001 Finland that either was struck slightly out of center, or had been a very slightly clipped planchet that the metal 'flowed' and expanded to fill the void under the enormous pressures of the minting die, resulting in the slight defect.
I have seen several coins like this.
btw, Bert, I sent your coins this morning! please let me know when they arrive!
Do you think this is counterfeit or rather a unsual mistake? And if the last one, do you think it can value more than 1 euro? It is quite old though...
I am not a coin specialist, but I've seen people taken out the inner part of the 1/2 EUR coins and putting it back upside down or otherwise mis-aligned. Guess that happened here as well, so no counterfeit but no special value either.
I think the way to do this must be to heat the outside edge of the coin with a torch (the golden metal ring) as it has a lower melting temperature, and will expand more readily than the central disc. the ring expands (theoretically) to the point where the inner disc falls out. it is then re-inserted upside down, and allowed to cool so that the outer ring will contract and grasp the disc again.
I've never tried it, but I'd imagine this must be the way it is done.
I've heard of several coins like this.