My parts book from 1961 shows 5 lined and 6 plain, (42 3195) so no difference in the plain plates according to this publication. So starting with a plain plate, alternating lined and plain gives you a top plain plate under the pressure plate.
PO could have read the wrong book...Haynes is particularly poor, diagrams show 6 spring and Triumph type 4 spring, but text refers to Plunger type and 6 spring.
Swarfy.