FYI, the Burkert gas solenoid on my SM has G1/4 threads. The best I can tell, G1/4 is the same as 1/4 British Standard Pipe (1/4 BSP). It appears that Burkert solenoids sold in the US market come standard with NPT threads and those sold in the rest of the world have G (BSP) threads.
Since it appears that your gas solenoid was added by an owner and not by Amel, it really could be anything. If there is a part number on the solenoid you might be able to Google it and find out. In any case, if possible you probably should physically check it yourself to see what the threads are if you want to buy a "drop-in" replacement solenoid.
SM #440 Cara
Deale, MD USA