I believe there are some more hardware differences.
The change was made from the MY43... to MY44... serial numbers and the later one should have been called 33220B but somehow that was missed, I guess.
I was looking for the option board for the 10 MHz input /output and noticed that there are two different boards, depending on the serial before or after MY44...
More interesting, the option board for the earlier models can be ordered from a distributor as an option but the same board for the later one is not sold through distribution, but instead a "repair" part from Keysight directly.
So, it is probably more that just the FPGA