Let's put it a different way. What makes you doubt that it is a Fisher blank?
Truth be told, if you like the rod, it doesn't much matter if you like the rod--any placebo effect of the name being long passed. But at the time you bought it, was Fisher well known and widely distributed in S.A. ? If not, would there be a reason for someone to misrepresent it, and if they did, what commonly available blank would be used instead?
Fisher was a go-to manufacturer for hobby builds and small shop builds for many years. But even in parts of the U.S., the name itself wasn't well known except to builders and their buyers. Telling someone on a secondary sale years later, "this is a Fisher" wouldn't add much to the likelihood of a sale or its price. A potential buyer would just judge the rod by his own preference for feel, action, and cosmetics.
Fisher made great graphite blanks but kept a low profile at the retail level. Today, its "vintage" graphite--and of course fiberglass--are as fine as ever--but with much more name recognition and cachet. Years ago, it would be a casual point to mention that a hobby or custom rod was a Fisher, so I don't see much reason that it would have been misrepresented--unless the market was completely different in S.A. Beyond that, the only way to know with near certainty is by comparison in hand with a known Fisher blank. It sure looks like one, but others had a similar appearance.
So in that way, if you like the rod, it doesn't matter anyhow. Identifying an old build as on a Fisher blank adds little to its value either.
Just to give the idea, I knew of Fisher 'glass and graphite since their time, built on one 'glass bank, several graphites, and saw many more because they were popular with friends and a shop owner I knew well. Fifteen or twenty years later, I got one 'glass blank and passed up a bunch of graphites at a salvage sale. I asked what kind of blanks they were, and the reply was just a casual, "I think they are Fisher," definitely not aimed at convincing me to buy or justifying the selling price, which was low anyhow.
I knew they were Fisher blanks--just 6 or 10--not just from appearance but because I knew their origin, how they sat unsold and wound up all but junked in storage. They were in remarkably good condition, but I passed on the graphites and got just one 'glass in a length and line weight I wanted. I won't say what I paid, and the graphites might have cost me a dollar more per blank, but it is only another decade or two later that anyone would think I "stole" them.
Probably a Fisher, probably purchased for a good deal, and probably a very nice rod.