1959 mexican 20 pesos gold coin

[kzf...@worldnet.att.net]

Gentlemen/Ladies,

I see people selling 20 pesos "types".
I own a 20 pesos coin which I always thought to be genuine.
I see very little difference in the coins - except that on the side
with the circle, the "type" coin has a distinct face inside the circle
while my circle doesn't appear to have one.
Am I correct in this being the difference?
And...how are people allowed to sell such similar coins?
Please indulge me if these are stupid questions and thanks in advance.
Ted