Blade etchings, like the kind Puma used/uses, can be destroyed or obscured over time by corrosion, sanding, hard use, etc. And clearly the OP's knife has seen some hard times.
I can't say for sure that the OP's knife is a Puma, but the mere fact that it no longer says "Puma" does not mean it isn't a "Puma". It's possible that if the OP looked very hard in the right place that he might see remnants of the Puma etchings.
Also, according to Puma's own website, they use a 5 digit numerical date code stamped into the guards of their fixed-blades. The OP's knife has a 5 digit number stamped into the guard, and when you enter that number into the date code search function on Puma's site it comes back just like the OP said, the second quarter of 1966.
If it
looks like a Puma, has a 5 digit number stamped on the guard like a Puma, and the number perfectly matches a specific date code in Puma's date code registry, I'd say there's a good chance that it's a Puma. At least I'd say it's strong evidence of it

.
Picture showing a Puma knife with guard-stamped date code.