The MG151/20 and 20x138 Flak color markings

I'm researching MG151/20 and 20x138 Flak color markings. I came across the following diagrams.
Aircraft AP munitions:

Flak 20x138 munitions:

I noticed the "bulbous" tip of the Flak munitions. I've seen other references/diagrams where that little step from the cylindrical part to the slightly wider tip is also visible. Here is one such other example:

I've seen plenty of photos of complete Flak rounds that do not appear to have this feature.

On the other hand, I've also seen an example of an API phosphorous round that *does* have the bulbous feature, but has markings consistent with the aircraft AP munitions diagram shown above (black with blue ring and white "Ph").

Is this "bulbous" feature something that can be present on both Flak and aircraft (MG151/20 or MG FF) munitions? Was it a result of a certain manufacturing process? Or perhaps the phosphorus markings on the API round I saw were wrong?

Thanks!

From my collection. I hope this helps. I do have copies of manuals on this ammunition if you need them. I thought pictures of the originals would be of more help.