Before you take any advice from anyone , make sure you get advise from someone who:
1. owns a U67
2. has experience cleaning it
Did you tell us what caused you to reveal the capsule in the first place ?
was it :
1. just dirty looking
2. does it make stange noises when powered while giving it a listening test
Does it have any discolored residue on it, caused by saliva, dust, cigarette smoke, especially on the dull matt side you described ?
In many cases if the capsule is cleaned properly but does'nt survive the process, the capsule was probably on the way out anyway.
Check the condition of the capsule membrane with a hand held maginfying glass to see if you can spot any surface defects in the caspule membrane, like pitting or gold flaking, lifting or any surface that is not smooth. Often , it takes a handheld maginfing glass to see the beginings
of capsule membrane deterioration. If all looks good then the capsule will
survive a cleaning assuming you have the experience to perform it. Its not terrribly hard, but you have to be extremely gentle. During the cleaning process, the water has to glide off the caspule membrane with absolute featherlike presence on the surface.
If you decide to clean it yourself, I have had good luck with a small lab style squirt bottle and distilled water. Very very gently allow the water to run off the capsule while it washes the small particles off. Afterwards, let it dry completely, and take note that if there is any pitting in the capsule membrane the distilled water may find its way into the capsule body. If there is any pitting you will be able to see it with a handheld maginfying glass. In fact the pitting, may be more visible after the distilled water wash. The best scenario is that your capsule membrane surface is smooth, with no surface defects. A dirty capsule when cleaned does sound better because the moving physics of the membrane material is effected by surface matter. Just depends how much surface matter is clinging to the membrane.
lastly , always store the mic with the head grille/basket covered with a clean plastic bag and rubberband to hold it on. This keeps the membrane from attracting dust particles when not in use.