I misread the title and initially thought that it would be about the DEC VT340, a graphics terminal that I used a lot back in the 80s. Interesting that product numbers were reused.
It's not a VT series terminal, but a Type 340 CRT vector display. The "Type" designations were used with early DEC products.
When you're in business as long as DEC was, I think that inevitable if you want the product numbers to be short and memorable.