Thanks!

7 stands indeed for the 7 ft. long (7½ ft. exactly) of the first P-10 models, but:
• Step-Van 7 is the whole official model line name of the P-10 from 1962 to 1980 and written that way in Chevrolet literature. The 1958-66 rounded P-20 and P-30 were then "just" Step-Van models;


• P-10, P-20 and P-30 are not the code of these vans, but their model name, the same way C-10, C-20 and so on are for the C-Series trucks (hence the P-Series I propose as a generic name);

(Excerpt of the 1984 Step-Van brochure)

• the 1965-66 Step-van line was made of Step-Van 7, (rounded) Step-Van and (boxy) Step-Van King models;
• the 1967-71 line proposed Step-Van 7 and Step-Van King models (boxy body, with the front face that it will have until the end of its production);
• 1972+: the 7 and King complements disappear on some brochures, but reappear from time to time;
• 1979 seems to be the last model year of the Step-Van 7 P-10.

I promised a post on Step-Van codes to rjluna2, but it sounds like a rediscovery of the whole story of these vans might be helpful to all of us