First, the description from the seller is not very good. But I'll try to ferret out a good guess for what the missing parameter is.
The 40pF is the maximum capacitance. There is a fairly wide range of constructions and minimum capacitances, but if we confine ourselves to a search for 6mm ceramic variable capacitors, and look for similar construction, the Sprague-Goodman GKG series is very close.
Note that the color codes match as well (yellow for 40pF).
It's only a guess that the minimum is 6.5pF, I would not be surprised if it was a bit higher, but if you design for 10pF~40pF you should be okay. The tempco should be specified as well- the Sprague ones have a -750ppm/°C coefficient.
It is a bit silly that the parametric search parameters at both Digikey and Mouser sort by the minimum capacitance. The maximum and a range or max, then min would make a lot more sense to an Engineer.