The largest of the three MIVA "spook" fuse holder (portafusile) insulator family from Italy. I have seen these in Clear and Aqua glass, but not yet in dark / emerald green. The two smaller members of the family are BT2V and BT3V.

Nora has one in Pyrex glass shown here: [id=81668111]

And another one in aqua, showing the BT1V embossing is shown here: [id=114046686]

This one arrived today, and I mounted some nice clean brass hardware from a damaged clear glass BT1V, I'll post a new photo soon.