The informations in those identification booklets Jörg is talking about are based on the knowledge gained through pre-war experiences, shot-down and examined examples and of course by secret intelligence work.
Thus any (wrong) information about prototypes and propaganda information may appear in those official identification prints.
First, in German writing "I-17" is the same as "J-17". (The written stroke "J" - like in "JG" e.g. would be longer)
The Polikarpov I-17 was a prototype with an inline engine from 1934 obviously thought to be operational
http://www.aviastar.org/air/russia/pol_i-17.php
obviously significating the early LaGG and MiG fighters
The I-18 might have something to do with the Polikarpov I-180 and I-185 prototypes with radial engines, obviously, and thus would mean mean the La-5 fighters. I am sure the Germans did not know about the LaGG, Jak and MiG designations after 1940 (LaGG-1 starting as I-22, Jak-1 as I-26, MiG-1 as I-61) and the situation in the unfortunate Polikarpov offices.
The "I-" was a producer-independant anonymous USSR designation comparable to the US "P-" designation.
Hope this helps
Roland