Ted
We do have different opinions on many things in the t206 set but it's not
an opinion that Plank is a 150/350 subject it is a fact.
150 only means exactly what it states only printed with 150 series backs.
Not only was Plank printed with a SC350/30 back of all the know examples
there are twice as many SC350/30 backs as there are Piedmont and Sweet
Caporal 150 backs.
In this Plank Gallery of the examples with back images there are 16 PD150
and SC150's and 34 SC350/30's.
http://www.t206resource.com/Plank-Gallery.html