Interesting! And it doesn't look like the
original sign made space for elevation and population. And yet at Disneyland they had been including that info on the train station sign since 1956, 15 years before WDW opened in '71. The first figure Disneyland put up there was a population of 5 Million in the summer of 1956.
Disneyland kept updating the population annually, and here's that sign in 1965, when Disneyland hit 50 Million guests during the Tencennial, just before Walt announced the Florida Project.
Since Disneyland had added the Population and Elevation to their sign in 1956, and updated the population figure regularly for 15 years before WDW opened, I would have to think the 1971 sign didn't have that info on it because they had no population yet. I would imagine the WDW sign was updated with population, and a symmetrical elevation figure on the opposite end of the sign, sometime around 1972 or '73.