Anything "Valletta" is pretty new to me.
I certainly did not encounter any when I started collecting Maltese glass, not knowing which company made what, and eventually pinning down first Mdina, then Phoenician and Mtarfa and finally, MDG.
Now, Valletta has appeared and needs added to that list.
If it had been going since '79, I'm sure I would have encountered it along the way.
But the images of Valletta glass posted by glass newbie are decidedly contemporary takes on the original Mdina Tiger/Marble designs. (I personally tend to describe the flat execution as being Marble and the deep colourful stuff as Tiger.)
I would imagine you are on the right sort of track, suspecting Mtarfa has become Valletta, Wolfie.
And if Mtarfa had gone under, perhaps going back to original (Mdina) designs of colourways might have been seen as more appealling to the market than the things they had produced themselves. Some of their stuff, I thought, was wonderful and I have a good bit of it, and while the intestine-y bottles and draped/gate sculptures are a bit weird, they are not weird how I like weird.