Ahh technically they are Forcella Italia...
There are a few different types, those look early-ish, similar tops to mine but different bottoms.
What diameter? Guessing 38mm.
Yes, they are pretty rare, mostly just sold in the after market AFAIK, expensive bling for the racers etc. I don't think they were ever put on a production bike, closest was the early 80's Bimotas which used something similar but a bit different and I think later than those ones.
Ceriani morphed into Forcella Italia which then morphed into Paioli.
As far as magnesium goes, only way to know for sure is to remove the lowers, weigh them (note M), then fill a vessel large enough to fully immerse them with water right to the brim, weigh that (note V1), then slowly put a fork lower into the vessel so it spills the displaced water out, then carefully remove the fork lower again, then weight the vessel again (note V2).
The calculate M / (V1-V2)
If it's about 1.7 then they are a magnesium alloy.
If it's about 2.7 then they are an aluminium alloy.
