Attached ROM images that were extract from HP8702B Serial#:3310A004xx 3GHz Opt.: None.
Device for U4, U5, U24, U25 : D27C010-200ns class EPROM.

EPROM (U4,U5,U24,U25) : Firmware Version V01.00 Jun.28, 1989
ROM Parts# 
U4:08702-80012
U5:08702-80013
U24:08702-80014
U25:08702-80015

Experimentally I put HP8753C V4.13 ROMs (down load from this site) into above HP8702B.
It worked correctly as 8753C w/o any modification. 
I think that it is possible vice versa. 
But did not check yet.
(Please experiment by a self-responsibility.)
HP8702B has the time domain function (010) default. 
Therefore 010 option appeared in the start up screen.

mmbfj310