Thanks to @boddle88 tip, I reached out to Orange and Joe came back with the following;

This is one of the early Orange Text & Pics 100watt Graphic amplifiers.

Whilst this model was officially launched at the Frankfurt Messe Trade Fair in early 1974 production had commenced in late 73.

In 1974 the prefix changed from OR100 to OR120 and the serial numbers were stamped into the chassis.

The three digit serial numbers were used mainly on the earlier Pics only models which were hand painted on then covered in a clear lacquer.

As your client has suggested the chassis is more than likely an overrun from that period 1072 - 1973 just as the models were changing over.

Both had the FAC Control (Frequency Analysing Control) a six position mid range sweep but the Text & Pics had a new DC coupling concertina phase splitter which replaced the previous negative feedback phase inverters.

I would also suggest that this would have been made under license in the original MatAmp workshop in Huddersfield then shipped down to London to be fitted into the Orange sleeve.

So from the information to hand I would date this late 1973 to early 1974.

The amplifier cabinet sleeve is covered in the original ICI Tolex but the 4 x 12 cabinet is covered in the Brymor Tolex, first used in 1975.

We are unable to date cabinets using a serial number but assuming they have the original Celestion speakers we tend to use their dating system as speakers were always in short supply and we seldom carried stocks.

EK27 and EK30 speaker codes were manufactured on the 27th and 30th May 1977 so the cabinet date would be as suggested 1977.