Help figuring out my 8648C Signal Generator?....

[TERRA Operative]

I recently picked up a HP 8648C Sig Gen and am trying to figure out exactly what it is...
It had the serial and option stickers removed (A semi-common thing here in Japan when companies liquidate old gear, because reasons) so I'm trying to figure out just what this unit is fitted out with.
Luckily the instrument stores the serial number in its memory, so I was able to use GPIB to read it out: 3426A00711

The three plug-in modules inside are a little strange when checking their part numbers... The part numbers don't seem to exist in the service manuals for later serial numbers that I have found for download (Although ebay has some paper manuals that cover my serial prefix), but they do exist when checking the Keysight Parts Finder website.

Details as follows:

A4: 'OPT REFERENCE' P/N: 08647-61046 -> Described in Keysight Parts Finder as 'Replaced by 08648-60042', which is 'MOD Assembly REF - 1E5' so I guess I scored the option 1E5, high stability time base.

A5: 'SIG GEN SYNTH' P/N: 08647-61050 -> Described in Keysight Parts Finder as as 'RF Synthe Assembly For 8648B' (I assume the B designation isn't exclusive to the B model as there are many parts shared between B, C, and D models?)

A6: 'RF OUTPUT' P/N: 08648-60116 -> Described in Keysight Parts Finder as as 'MOD AY-OUTPUT'.


Does anyone know why these part numbers are different to the manuals (Both Agilent and HP branded manuals)? Maybe I have an older unit and they are just older part numbers?
Also, does anyone know what 'Mod AY' is?