As stated by others, your stamp is perf 12 and cannot be #500. However, I think that I know where your confusion comes from. I have many of the perf 12 #406 that show a complete toga rope and button, but they are relatively weak and thin compared to #500:
I also see many #425 with similar thin line detail but weaker still than the #406:
The #500 has to be perf 11 and the top of the toga rope line and the button is strong. The surest way of identifying a #500 is that the button looks like a doughnut. The circle is quite thick:
Here is a perf 11 #499 that has a nearly complete top line of the toga rope and full button, but isn't strong and thick enough to be from the intentionally modified die used to create the #500:
Many #500 will not have as strong of detail as my example, but will be significantly stronger than my #499 example.
Here's one that Clark has listed on eBay
. Note how the outline of the button is still quite thick and how the area pointed to by the arrow has pretty clean and defined lines below the toga rope. Not all #500 positions nor printings have clean detail there, but it is a strong indicator if you find these clean "digs":
Here's an example of a block of #500 that doesn't have strong impressions, but you can see how the button outlines are substantially thick: