Ethiopia 50 c. KM# 37 and 37a

Hi, friends

Do someone know how to identify these coins?  As I understand difference just in Silver %
Same year, same weight, same design. But different KM#.  
 I don't understand - if in other coins, where actual visual differences exist, they write - "planchet / die/ thickness/ rim/ edge/ - varietes exist"  in same KM# , while here, in Ethiopian silver  50 c,  - there are  two almost same subtypes under KM# - 37 and 37a, and nobody can identify which is 37 and which is 37a

OK with Krause, -- sometimes their stupid reports exist, but, perhaps someone of you know a secret how to identify these two subtypes without jewelery expertise.    
regards
David

https://en.numista.com/forum/topic23703.html

Quote: ZacUKhttps://en.numista.com/forum/topic23703.html

Ahh, thank you for refer.
  but I don't understand a stuff with ring. What ring?

I know that silver leaves a black or dark gray trace on the skin, and if you are so lucky that have both 700 and 800 percent coins, 800 will trace more black than 700. Perhaps it will  work...  I will try to compare a trace of my Ethiopian coin and other coin with  700 or 720 silver, and 800 silver...  must to find in spares.

 Thank you...

'The .800 would have a ring when dropped closer ...' ring means sound - different % make different sound.

Quote: ZacUK'The .800 would have a ring when dropped closer ...' ring means sound - different % make different sound.

now understand.   thank you !