There are several examples if not more circulating in the market of fake inverted watermark Hong Kong stamps.

How do they do it?

The stamp paper is made up of many layers and with expertise, the layers can be separated. The watermark on the Queen Victoria Hong Kong stamps is a single Crown with the letters CC or CA below. The watermark is in most cases found in the center of the stamp. Most unfortunately, the size of the Crown and the CC or CA is about the same thus the faker could separate the layers of paper, switch it around and regum the paper back in place.

How can one tell?

1. The paper tends to be thicker or stiffer because of the extra layers of gum. The time honored "finger flick" test works.
2. The postmark tends to be less crisp.
3. Most reliable way is to check the perforations with a 30x or more microscope to see separation of paper layers.