I seem to remember us having a discussion about double-pole fusing and when the NEC disallowed it. I can't rememebr what the answer was though -- Late 1920s/early 1930s?
It was illegal to fuse neutrals of multiwire systems by 1923, possibly several years earlier.
As for single-pole branch circuits, or two-wire services such as this one, at some point it was thrown to the AHJ to decide. Many of us have had the experience of ordering a disconnect and having to specify that it come with a "solid" neutral (like, what other kind is there?). This is because, in the '30s, there were two types of equipment, those with solid neutrals, and those with fused.
I have every NEC from 1940 to the present, and I also have a Terrell Croft volume containing the entire text of the 1923 NEC (sort of an early "Handbook"). The '23 NEC clearly disallowed fused neutrals for multiwire systems but allowed them for two-wire circuits.
From 1940 to 1953, this is the AHJ's call.
The 1956 NEC contains language similar to today, i.e. 240.22 in the 2002 NEC.
My sense is that fused neutrals became markedly less common after the mid-30s. I still have one of those 2-circuit, 4-fuseholder porcelain cutouts. I found it in a long-abandoned barn in Connecticut ten years ago. It was enclosed in a manufactured cutout box, and the wiring was all Type AC cable, I wanna say TW insulation although I can't remember. If it were TW, that means probably 1948 or later. The neutral fuseholders puzzled me to no end!
