That is a light mount bracket. The light has a mounting fixture that slides onto that vertical squarish section.

-----

it is placed upon the right blade because the cycle is designed for use in a land where traffic flows on the left side of the road

the brazed-on fitting on the fork blade is referred to as the "lamp boss"


-----

Required by law for any cycle intended to be ridden on public roads in the UK. But if the bike/trike/cycle was made strictly for 'competition' (or for export out of the UK) then the lamp boss could be, would be, left off. Left right off

It's a lamp bracket, designed to fit one of the old Ever Ready lamps.

The lamps ran on two D sized batteries which lasted for an hour if you were lucky, and illuminated a small patch of road about 6 feet in front of you. The spring loaded clamp on the lamp wasn't great either, so on occasion if you hit a bump the lamp would spring off your bike and clatter into the road, popping apart, shedding the batteries and frequently breaking the bulb too.

There was a rear version too, which had a bolt on clamp around the seatstay. Same battery life, just as dim, but at least it didn't ping off the bike.

Brings back memories of evening rides in the 80s - we rode in a line with the leader being the guy with a working front light and the man at the back having a working tail lamp. Happy days, but I love the rechargeable LED lights I use now!

The light, that is; not the boss.

And only when ridden (no intent required) between sunset and sunrise.

(And the above is only for Britain; other bits of the UK have their own Road Traffic Acts and regulations that may differ.)

I think OP needs to get out more.

I have similar bosses in the same location but on both sides on my Mooney forks. For LowRiders. I asked Peter for LowRider bosses when I ordered the bike and he said he would not drill a fork blade. 11 months after my fork breaking and near killing me, I didn't argue and used the Blackburn U-bolts. 4 years later, I had crashed the fork and ordered a new one from Peter (with tweaks that might improve the ride), Ask again re: the bosses. He said he was willing to go the light bosses because he wouldn't have to drill. I said yes and he brazed on English bosses considerably sexier than those Carton bosses. 5/16" bolt so I have to enlarge the rack holes but they have worked flawlessly.

And a comment on the lights BITD. I bought the then common generator light and mounted it on my new Peugeot UO-8. Ridign home at 9pm, I passed a single parked car relatively close because I knew there was a car a ways behind me. The parked car was dark and under a streetlight so I could not see inside. Door opened and I hit it. (The door was history. I walks away with just two perfect 1" bruises on my thighs form the bar ends. Crooked bars and brake lever.)

I realized afterwards that the light was not bright enough. It allowed me to feel secure but offered none. I took it off and rode without a headlight for 25 years until there was a good halogen light. The entire time I rode with a light strapped to my ankle. Drivers saw that, even BITD when it was a 2 C-cell job. (There is no other light that goes up and down. Different attracts attention.)