The Concert amp came into production in '59 and was offered, from '59 till '63, in Brown Tolex with 4-10 inch speaker 40 watt combo amp.
In '63 the same basic amp(4-10s, 40 watts, tremolo) was offered in black tolex, then discontinued in '65.
When Fender came out with the "II" series in the '80s they brought back the "Concert" name, from '82 till '87. It was offered as a 60 watt amp with reverb and offered with 1-12 inch speaker(Concert 112), or 2-10 inch speakers(Concert 210), or 4-10 inch speakers(Concert 410), or a head(Concert top) and various cabinet combinations.
Then from '93 to '95 the "Concert" name appeared on the Fender "ProTube Series" amps. This time as just a 1-12, 60 watt reverb combo amp.
The Fender "Concert" was never offered with a single 15 inch speaker or produced during the silverface era(the '70s).
So if your looking at a Concert with a 15 inch, then either the baffle board has been changed or the cabinet has.
If it has reverb and channel switching then it is from the '80s or '90s.
With that all said, I have played a '61 Brown Concert through a 15 inch JBL D-130 and it is a beautiful thing to hear.