I would agree with you. I think perhaps the board could have the 3.3V circuitry powered directly by the PSU, or via a regulator. I'm not that familiar with the 750, the 330/350 were 3.3V socket 5 Pentium systems based on the 430FX chipset. I don't know for sure but the motherboard looks very much like it's an Intel OEM one.
The first time I encountered this connector was on the Intel socket 5 430NX Premier PCI II. This is the socket 5 version of the socket 4 Premier PCI 'Batman' motherboard. However It is also present on the 5V Batman motherboard. The manual for that states that it exists to maintain strict compliance with the PCI spec. So 3.3V has to be there but AFAIK unused by any PCI card until the late '90s. In today's parlance it is a "5V PCI slot".
Here is what I think happened. I don't have any proof, just a guess. PCI and the Pentium were introduced in '93, with PCI having 3.3V in the spec. Shortly after the Pentium went to 3.3V. If you have 3.3V components it would make sense for the PSU to produce that voltage. However you currently have the AT PSU connector as an established standard. So why not design a board that as an interim can do 3.3V via a regulator, for maximum AT comparability, but could also work without a regulator, and have 3.3V come right from the PSU. Then go full 3.3V and specify a new PSU standard. This line of thinking did exist because they came out with ATX in '95 I believe. But then, the plan kinda went off, as the MMX CPU's used the split voltage, additionally you had the clones super socket 7 with all sorts of voltages.
Another board I have is the Intel (430FX) Advance/AS. This is an AT board with some integrated video. It was found in Dell's p75T and p90T. In the Dell, instead of the standard AT connector it has an ATX-like connector, that's not pin compatible. I had originally thought it was a DELL being DELL. However It does look like an Intel idea, as non-Dell variants have the solder pads for the connector. If you remove the connector, you can solder in a standard AT connector.
I *think* that 6-pin 3.3V connector is the same pinout for that in the ATX 'AUX' connector.
Also Intel, had been adding additional PSU connectors to their motherboards since their first 386 motherboard. They've tried more than once to update the AT standard, and by extension motherboard design in general. You can take this FWIW, but all these little oddities look to me, like the pre-history of the ATX standard. There are some additional modifications Intel made, like with their xPress server platform. This is currently my best guess as to all this, but there is very little to go on really.
