[Einar72]

Okay, the 6BL8/ECF80 first. These tubes were cheaper than their U.S.- made counterparts like the 6U8/6KD8/6EA8/6GH8. Both Motorola and Fisher used those brown-painted imported resistors under the chassis back then as well. My Hallicrafters S-118 has two 6BL8/ECF80's marked on the tube chart. Moto, Philco and Magnavox all were sporting plenty of Japanese-made tubes "under the hood" come the end of the tube era.

I have a suspicion as to why Heathkit embraced the 6EA8 being that they were left "standing at the altar" in huge quantities by the major U.S TV makers once the cheaper-made 6GH8 came to town

The 5BC3 was a last gasp cost-reduction attempt for the 5U4/5Z3/5X4. The Novar-button based envelope was mucho cheaper as it eliminated the phenolic base with its pin-staking and bulb-cementing operations that cost extra $.

The 5Y4 (and its 5U4-thwarting cousin the 5X4) were a bold attempt by Philco and occasionally Zenith to force factory-service-dealer loyalty by being incompatible with the get-'em-anywhere 5Y3 and 5U4. Don't forget the 5Y3 crammed into a Philco Loktal package known as the 5AX4!