I had similar deposits on the valves of my 216 that had been sitting for years before bringing her back from the brink. My thoughts are similar to Denny's in that it's probably lead deposits. I did what Denny suggests and used the wire brush on my bench grinder to clean them off.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.