There should be "bracing" inside if they are a "1.5". The presence of a metal K-600 horn would indicate that they are 1's. The K-52 midrange driver however, was used with the K-601 (plastic) horn. There should be: (a) rear panel bracing (3/4" birch strips), (b) motor board bracing strips, and (c) the "shelf" below the woofer.
The boxes, once the motor board and rear panel are installed are very, very sturdy. No additional bracing is/was necessary in that the construction was a very high quality birch plywood. In later models where MDF was used, the MDF is of high quality and the motor board and rear panel are rabbeted and fit into slots making the assembly extremely strong.
For detailed data on the Heritage series, including Cornwalls, for serial numbers, drivers, horns, finishes, etc. see:
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