...and from a discussion in another topic (perhaps when discussing Dan's early 500 set), it was pointed out that we can learn a lot by looking at the bottom plate.
In this case, note the hex head on the left side of the photo (opposite the handset cord entry). It holds the bracket for the tube mentioned in the chart.
(Often these were removed if sets were converted to 500Ds. Then we usually see some scraping to the paint around the hole, telling us the tube had previously been there.)
[added second photo]