Drain-screw modification - how is it done?

This is a question for Scotch

Can you tell me how the socket head cap (allen bolt head) was attached to the early type carb drain screws? Was it regular plumbers solder or were they MIG/TIG welded or brazed? (I don't have access to the last two)

I'm cleaning my spare set at the moment with a view to getting them to a working condition and I've just had one hell of a job getting the drain screws out and would prefer something easier for adjusting float levels in future

Looking back at scotch’s epic carb postings, I did see that he mentioned silver soldering a socket screw head to the drain screw.  

Have they stood the test of time?

Socket-head to Pilot screws are attached with regular solder.  The socket heads for the Drain screws must be "Silver" soldered (brazed) requiring a higher temperature
When done correctly the do stand the test of time.

Thanks for feedback. I decided to progressively drill out mine to 3.5mm and then tap to M4.  The Allen screws were then cut down, screwed in, and then silver soldered as Scotch suggested. I think this is the first time I've had any metal glowing bright red/orange since leaving school 43 years ago   Managed to get enough heat into the metal with my plumbing Map gas torch.