Hi Bob.
If I remember correctly, the Americans experienced a high failure rate with their early E1148, so dropped the 6K7G spare valve in the outer position of the Spare Valves Box in order to carry a second E1148. On the Canadian side, we never had that problem so just the single, inner spare E1148 was ever carried in their Spare Valve Boxes. The E1148 is a required Valve in any North American made Mk II or Mk III 19-Set.
The American Spare Valves Cases used black, cotton ‘socks’ to cushion several of the spare valves, if the case got a hard, lateral shake. To accomplish the same thing, some, but not all’ Canadian makers of the Spare Valves box used a long strip of black roofing felt wound among the valves to cushion them. You can see this as a heavy black line printed on the valve identification chart glued to the pad in the lid of the case. I think some makers may even have used two strips of the felt.
David
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