4.9Ghz Link "1 2 3 4 5 6 7 8 9 10 11 12 13 14 15"

Why can you not set a 1x40 channel in the 4.9 block?

The FCC rules are written for 20 MHz channel width only.

So when you are on a 40Mhz channel the center freq becomes the mean of the primary and secondary?

I was under the understanding a 40Mhz channel was two 20Mhz with separate centers aggregated.

In 802.11n, there is a primary channel and extension channel that form a 40MHz channel, but 802.11ac treats a 40MHz channel as one channel with one center frequency. That is inconsequential though since the FCC regulations allow for aggregation of smaller channels up to 20 MHz in the safety band, but not larger. For that reason, you can select two separate channels of 20 MHz (with B5c), but not a single 40 MHz on either B5c or B5-Lite.