Apart from the Bakelite bases on octal tubes, I figured there should be no problem shining a light up through the glass envelope. Come to find out that some of the tubes with Miniature 7 bases have an electrostatic shield (?) across the bottom that pretty well blocks the light.
This 6BJ6 has a neatly trimmed octagon:
The shield plate, if that’s what it is, doesn’t have a standardized shape. This 6CB6 sports a simple square:
The Box o’ Hollow State Electronics contains one 6BE6 tube (a heptode with five grids connected to four pins) without a shield:
Yeah, those pins are rather grotendous.
And another 6BE6 with a semitransparent smudge not connected to anything else; it would look accidental if it weren’t inside the tube:
All the shielded tubes are pentodes, for whatever difference that makes.