The AO-28 preamp does have an RCA phono input, I've never used it though. It could come in handy when doing an A/B comparison b/t the vintage organ and a new clonewheel organ.
I'm just taking a wild guess here, but I'd say maybe Laurens Hammond stuck that phono input on there so you could add an external audio device and play along with a song (whatever they had in the 30's and 40's). Since that was the time when Hammonds were distributed with a tone cabinet (stationary speakers) that would probably be a logical guess.