Try to contact Agilent, in manual they are no so strict and maybe they will populate the OCXO instead of board swap.
Manual says : order 53200U-010.
If you needs certificate I cannot help you.
OK, found it in the component level manual:
"Retrofitting to Option 010 (Ultra-High Stability Timebase) must
be performed by technicians at a Keysight Technologies Service
Center. Refer to the list of Keysight Sales and Service offices in
the Appendix for the office closest to you.
NOTE To order the Ultra-High Stability Option 010, you will need to
place an order with Keysight for 53200U-010 and identify
which counter you have (53210A, 53220A, or 53230A)."
Those cowards don't want to sell the OCXO as a spare part?
