[78-L] 78 grove spacing

[ciadelgramofono at yahoo.es.invalid]

 Very interesting, and thanks Mal, for this enormous book!
First time I owned one of those DGG 'Variable Micrograde 78' records was in the 80s, a single 12" record with the Dvorak's 'Neue Slawische Tänze'. It certainly lasted much longer than a standard 78, and I believe the groove is finer too, for it can be played with an LP stylus. Later I traded it for other records (hard student-with-limited-budget times).
Recently I've acquired another one, and I've also noticed it's longer duration (exact data not at hand). It's also very tough, not sure if the material of the record is current shellac or a more advanced plastic, not exactly vinyl... The surface is very soft, but the record is heavy as a shellac one. 
BTW, in a different fashion, I've also noticed that late 1930s Vocalion records also exhibit a greater groove spacing towards the end of a side. Very characteristic of them, not for the red Columbias, but the blue Vocalions, and seems something different, not related to the musical contents of the grooves.
Inigo