Since you are in Lima why not go to the Library and look at the postal Law 17050. With a little digging you should be able to find out how many stamps were overprinted. If a few then rare might be appropriate. If a lot then scarce is more appropriate.
I had an issue of how many of a particular Colombnian stamp was issued. When I was in Bogota, Colombia I went to the library and found out from the postal laws and records. I did need help with Spanish so I took my wife's cousin with me.
The reason why you can't find them for sale on the internet could be their low value. Stamps of low value rarely sold separately, mostly in collections. The only way yours could be rare if there is some sort of error involved, in design or overprint.
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