[Arran]

 The 6A7 was the predecessor to the 6A8, 6A8G, or 6A8GT, electrically the tubes itself has the same specs except for one detail, the inter-electrode capacity is definitely higher then that of a 6A8 or 6A8GT, perhaps even a 6A8G as well. In this case it won't matter much, the 39-30 only has two bands, and the shortwave band runs from 4.9 mc to 18 mc, so the inter-electrode capacity is NOT an issue. I don't know is these sets were all equipped with 6A8Gs from the factory or whether some were built with 6A7s, Philco was notorious for changing tube layouts in their sets if they got a deal on one type of tube verses another, if the 6A7 socket is held in wite screws and nuts then a serviceman changed it, if it is held in by hollow rivets it may be original.
 Even so the shortwave band is more or less just a token feature on this set, with a pentagrid converter, no RF amp (tuned or untuned), or an extra IF amplifier stage, the set is not going to be very selective or sensitive on shortwave anyhow, to say nothing of the frequency spread. The addition of a 76 tube for AVC seems rather pointless, I think the sales and marketing department had the engineering department add it just so make the 39-30 chassis add up to six tubes, rather then five, to claim it was a better set then a 39-25.
Regards
Arran