[Test-Equipment] HP 34401A serial numbers

[DaveC]

Have 2 of these with significantly different serial number format:

MY4100xxxx

(firmware rev. 09-05-02)

and

3146A6xxxx

(firmware rev. 11-05-02)

Is there a date code here? Did the format change at some point?

I'm new to HP and this model so any info regarding the series, 
updates, etc. would be much appreciated.

Thanks,
Dave