I think you can be pretty confident that it's a deliberate (amateur after-market) modification for small chainring clearance. But if the seller claims it's original to the bike, here are some pictures of the same model frame where you can see clearly that there's a slight indent, but nothing like what you've got and nothing visible on the inside of the stay:
https://picclick.com/Nos-Vintage-Spe...018114285.html