Odd Looking Surface

I have the 5 mark version of this coin with what I would describe as the same funny surfaces, I always thought it was due to an old cleaning, but otherwise it is in high XF to AU condition. I will be interested to hear if anyone has another opinion.

Is that rough looking area on the obverse (6 to 9 o'clock) tacky/sticky in any way? Or is it just light areas of wear showing through?

I've seen quite a few coins that had either been cleaned and retoned or which simply gathered dust and debris that combined with oils from people's skin to develop a dark appearance as seen on the reverse (eagle) of the coin you showed.

Whether relevant to this coin or not, it may be interesting to note that collectors have put all kinds of things on their coins in the past with the intent of protecting them (though seldom with the desired result).

You mentioned lacquer and, in fact, many coins were lacquered in the past. In fact, I think this practice might still occur among collectors of old medals.

This is by no means a recommendation. I just thought the information interesting and worth sharing.

Neat!