Just some helpful advice for next time you look to buy one of these old Toyota pickups.
The early Toyota trucks (pre VIN) had a manufactures plate attached to the inner drivers side fender which provides the model #,engine size, trans/axle type and most important the frame ID number. Unfortunately, these plates are usually held in place by two Phillips head screws, making it easy to switch plates.
To verify your truck is legitimate you should check the drivers side frame rail just above the lower control arm. The frame ID number will be stamped there. The plate and frame numbers should match. If they don’t, you may be buying something that’s stolen.
I’ve included two pictures from my truck to show you what to look for. I’ve highlighted the frame stamp to make it easy to read.

 

Mine's riveted on the fender

 

Still easy to do. Drill out the two rivets , swap, then rerivet.

 

sure makes sense - but the rivets on mine are "different" than the pop rivets I'm used to and to me would be obvious they're not originals -- IIRC they are hollow in the center like some kind of a tubular rivet, not with a shank broken off in the middle like a normal pop rivet. But yes, you can drill them out easily

 

What I have found in the past is usually once the tags have been switched the thief will shoot some matching spray paint around the inside fender making sure the overspray covers the new rivets.
The hollow rivets are still available as well as the solid ones you mentioned. You’ve got a good eye.
That was one of the first things I looked for, back in the day.

 

I've heard of that one before from a CHP officer among other BS people do with VINs and ID plates - I bought a K5 Blazer out of state and I think because it was a smog-exempt diesel, the DMV wanted me to get the VINs and engine type verified before registration. The guy was VERY thorough, I thought it was kinda obsessive, lol.

 

The trick they used to do with the hollow rivets was to slide a 4P common nail through the backside then grab the pointed end with a pop rivet gun and squeeze. Made it look (almost) factory.
When in doubt, go with the frame #.