In the MSFS2020 SDK documentation, section engine variables, it is stated:
RECIP ENG PRIMER:index
The indexed engine primer state. Bool
Actually that variable uses a Position or Percent Unit as correctly shown in the ASOBO_ENGINE_Primer_SubTemplate in this statement
<ANIM_CODE>(A:RECIP ENG PRIMER:#ID#, percent)</ANIM_CODE>
Also in the Simvar Watcher the available units are listed wrong