Malaya Penang Stamp - Overprint Or Postmark?

Hi all. Hoping there's some Malaya expertise out there...

This stamp has been in my collection some time. I initially disregarded the black 'Penang' inscription, believing it to be a part of some postmark. However, when I look on the back of the stamp, it's very visible as an embossed effect. I can't really show in the scan of the back - you can see a little of the black on the white paper, but in the flesh, it's really well impressed into the paper itself, showing as a relief on the flat stamp, like an overprint would be. Postmarks, to my knowledge don't impress into the paper like that, so, is it an overprint? And if so, what's its purpose?

it looks like part of a roller cancel. you can almost see the top of the date on the lower left perfs.

like this.

looking at the example I posted above, you can see the edges of the lettering blocks (what looks like [ and ] around the letters), indicating that the roller was applied with some serious pressure.

Thanks Kuhli - love your cover by the way! It's undoubtedly a postmark then, but I am amazed quite how much the letters have bitten into the stamp.

They make nice little curios - I wonder if other Malayan states had equivalent postmarks. Could make a good little sub-collection.