Thanks guys.
The 'blemish' part is also something I noticed. It's some sort of surface dent or something, I don't know. This is as close as I could get with my camera, I should get a magnifying glass to really be sure. When I cleaned the heads as well as I could with cotton swaps and some head cleaner it didn't feel as a gab, that's all I can say...All I felt was the relief slot.
Also, I can't get them any cleaner then this... I reckon the dirty parts I see is oxide?
This might be a dumb question but what exactly should I measure so that I can determine the depth of wear?
I saw on reel openreel.net that new heads are 6900 yen, which is about 40 euro (or $70). Would it be a waste to search new heads instead of relapping these old ones?