Si non potes reperire Berolini in tabula, ludens essetis non WIF.
Hey, get your stinking cursor off my face! I got nukes, you know.
The problem I don´t own "Convoy in flames ".
No worries. The WIF rules available on-line (the 2004 version, the latest available, can be downloaded from the WIF page on BGG, or from ADG's website) include ALL rules from ALL expansions issued before 2004.
So, 22.4.19 is the "Convoys in Flames" rules, and Walther subs can be found there, at the very end of the rules.