One could say its a direct successor if, IIRC, the 3450a was top DVM in the HP line back in 1970. However, I don't think there was an HPIB bus then so 'system' use would be whatever ad-hoc network one used to control these devices. Also, this DVM was hard wired, rather than use some early microprocessor.
But my criteria considers the existing top DMM today, the 3458a, where did its technology evolve from? What models preceded it that help make it what it is today?
Per the HP Journal, the core of 3458a, its accuracy, began with the 3455a.
Also, the 3455a was among the first microprocessor driven DVM that used the HPIB bus and had 'Math functions' [ e.g. stat functions etc., ] that are standard in the 345x series: 3456a, 57a, and 58a.
Earliest? I would argue the 3455A is a direct successor to the 3450A. Which might be related to the 3440A plug-in DVM.