1979 is a tricky year as it was the intro year to the 4gtronic so there is a bit of a mixed bag of applications in its intro years.

In my w124TD, under light load it starts in 2nd. Floored from a stop, it starts in 1st in D. Shift to 2 and back to D, it starts in 1st regardless of load. If the load is light it upshifts quickly, if heavy into it will stay in 1st to max shift point. Shift into 2 and always starts in 1st regardless of load. In D, with a heavy cargo load in the wagon, a hauler, it starts in 1st even if the pedal is only half way.

This would lead me to believe that the starting in 1st on some models is controlled both electrically via the kickdown switch, some kickdown activation thru the shifter lever, and pressure circuit controls in the valve body.

This would make sense given how flexible the cars were designed to be. Think of all the Euro factory hitch towing options, sedans, wagons both hauling and towing etc. Could you imagine towing a pop up camper to the Swiss Alps with only 2nd start? Just my opinion and observations.

One last test you can try, is use the handy dandy shift point indicators on your Speedo between the mph/kph tracks. At WOT, your car should shift at those points if in propped form.

Some decent info regarding the trans, varriations, models and dates:
https://en.m.wikipedia.org/wiki/Mercedes-Benz_4G-Tronic_transmission

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

If this discussion is restricted to 1979 US model cars, then the "4G-Tronic" box is not in play. The 722.3/.4/.5 series of transmissions did not come to the US until the 1981 model year.
In 1979 the autoboxes in the US were the 722.0 for V8s, and the 722.1 for 4s, 5s, and 6s.