I also wrote Georgena Terry directly about all this. How great for her to reply to me. Here's what she says:

"Thanks for taking time to email me about the wheel sizes. Here’s the scoop on the wheels:

In the beginning, which was around 1984, the 650c tire wasn’t on anyone’s radar. The market was familiar though, with the 24/700c configuration. Not only was I using it, but my initiative came from Bill Boston, a custom builder in Connecticut, who used it on all of his small bikes for both men and women. Your reasoning about the design is correct. It allows the builder to lower and shorten the top tube for smaller riders. The small front wheel is lower than its 700c brethren, so the head tube can be lowered, allowing the top tube to be lower.

The 650c wheel addressed the issue some riders had with the appearance of two different wheel sizes. (I heard one person refer to the design as a “circus bike.”) 650c works well for that rider who may be on the verge of having too long an upper body for 24/700c, but not long enough for twin 700c. I’ve found that even “tall” women prefer a shorter reach to the bars than a man of the same height.

Symmetry was first introduced around 1986 and had a long history. It came into being during the 24/700c era and carried on through 650c. That’s why the same model has both configurations. It just depends on when it was built.

BTW, I still build 24/700c for certain riders. As for 650c, I’ve replaced the 650c size with ISO 559 (26”) because so many more tires are available and the diameter of the tire still allows me achieve a proper fit for some riders.

This video series I did on frame design may shed even more light on things:

Frame Design Videos | Hand-built Steel Bicycles for Women by Georgena Terry

Thanks again for writing. Please let me know if you have more questions.

Best, Georgena"