Ray,
I don't think it's a modification.
It must have been manufactured this way in Japan for their armed forces.
It reminds me of the Soviet era later production run US-9 receivers made in the USSR which are based on the BC-348, but there were several mechanical, component and circuit differences, but with the same dimensions, front panel, dial arrangement and power/control connector. In fact, I have my US-9 (1968 production) mounted on a BC-348 shock mount, and even the connectors mate properly.
Also, the Soviet R-807 transmitter (mine is 1964 production, but its spares box contains tubes with 1972 manufacturing date) which is based on the ART-13 has the same dimensions and front panel as the ART-13 and fits the ART-13 shock mount. A lot of mechanical parts inside don't look like the original, the wiring and components are also more modern than the ART-13.
73, Meir WF2U
Landrum, SC