Good question. Typically I look at the Afrikaan language portion to decide, but since that is missing you have to find some other missing component. I am not aware of one, but I have uploaded both options for you. Study them and maybe you can spot something that will clarify the situation. As I compared the two, I noticed a color difference, and a variance in the detail of the overall impression which makes me thing your stamp is Scott 53a.
The first one is Scott 37 (no hyphen between SUID and AFRIKA), the second one is Scott 53 (hyphen between SUID and AFRIKA).