My first comment is that these speakers are in amazing condition & an absolute credit to their custodians over their lifetime. However, I do not believe the driver line up is 100% original & I say that only as a comment & not as a criticism as I would be happy to own them.
The Concerto was the premier Kef model over a period of some 10 yrs, late 60s to late 70s & during that time Kef revised & updated the drivers & Xovers which included but, not limited to, improving their power handling. The style of grille cloth, the xover mounting, the T27 (tweeter) & B110 midrange are of the type fitted prior to the update but, the B139 bass driver is the later version. KEF introduced all the revised drivers together.
The later T27 is quite different looking & also has wires exposed horizontally each side of the dome, usually blue & black tho sometimes white. The later B110 has a slightly different roll surround which extends a little further onto the cone & the metal basket is painted all black. The xover on the later version has the component side visible not the back of the pcb as these have. The B139 that was used with the drivers fitted here did not have any corners & was a completely smooth oval whereas this unit clearly has 4 corners & is obviously the later version. You will note that the tweeter & midrange are very neatly routed in flush with the baffle where the bass does not appear to be. My belief is that the original B139 has been replaced with the later version & I wonder if both have been done. Anyone who inspects them in person should be able to see if the bass units are routed in flush as it is not really clear in the pix.
Might be worthwhile replacing the other components with the later versions which are often available s/hand or new from Falcon Electronics in the UK.
Either way I'm sure the new owner will be very proud of them & if they were here in Adelaide I would be looking to replacing my, nowhere near as pristine, models with these.
Forgot to mention the later Xover is coded DN12 & is of the ab (acoustic butterworth) type. SP1004 is the older version.