The price on the card doesn't necessarily mean the price that was paid. It could simply mean the owner hadn't identified it yet, but narrowed it down to two possibilities. Or, it could have been added to deceive a potential buyer. That stamp is in very poor condition and I bet wouldn't sell for more than 5-10% of a decent copy, which in all probability is plain old #19.
To identify it properly, you need a catalog. Once identified, to get a going price try
ebay and reduce the value properly due to the poor condition. If it really were a #19a you'd need to get it expertised. Even so, it would be nearly impossible to determine it was genuine as mentioned above.