Thank you for your interesting info about them. By instinct I agreeed that ichang papeda might have longer thorn though I never had an actual comparison. I went to a Vietnamese grocery store here and they also sold kaffir lime as well. Those kaffir lime they sold were ranging from 4' to 6', they did not appear to have very big thorns. Unfortunately, they did not carry any ichang papeda beside the kaffir lime to study.
I guess it is nearly impossible to tell until they have fruits. The fruits of ichang papeda would have bumpy stripes in the vertical direction while kaffir lime have non-pattern bumps or wavy bumpy strips all over its fruits.
I read several sources online about the history of ichang papeda, and some sources mentioned the use of ichang papeda fruits as lime substitute. Some other sources also claimed that ichang papeda in some rare occasion produced fruits that has the quality very close to lemon, isn't it true?