Your best bet is to get the EIA codes off of each of the transformers, and go with the latest date code. There should also be a code on the basket of the speakers (assuming they are original).
See the example below from my late 70's Super Twin Reverb.
"When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top
"The Twin" was made 87-94.
The "LO" prefix to the serial number indicates it was made in Lake Oswego, Oregon at the old Sunn plant which Fender purchased in 1986. FMI closed the facility around 1992 so the amp's build date falls somewhere within that window.
"When injustice becomes law then rebellion becomes duty."