Help identifying characters on a memorial coin I recently got my hands into this coin

No idea how it got here, nor where it came from, I've tried to identify the text with some chinese speaking friends but we are unable to fully comprehend.
So far we guess that top left character is likely 畢
And bottom right character is likely 余
The other side of the coin:

As you can see there's definitely no &quot;value&quot; on the coin which we guess together with the fact that doesn't look like a chinese old coin, it's more like a memorial coin for some kind of finished study.
Thanks everyone!
Edit: Seeing as after over two years we still havent figured out much and this question still drives some attention I've decided to take a high resolution scan of the coin. I'm not fully satisfied with the quality so I'll leave the old images aswell.
I can only recognize two of them.
？砉
换？

砉 might be
换 definitely