That numbering scheme is used by Amidon.
Powdered iron toroids have a "T" prefix, while ferrite has "FT".
The next number is core diameter in 100ths of an inch. The
last, as you discovered, is the "mix" that describes the material.
Futher information.For
powdered iron cores, #2 mix (red) is rated for 2 - 30 MHz,
and #6 mix (yellow) is rated for 10 - 50 MHz. In practice,
#6 cores can be used on 40m as well. So if you only want
to cover 40 - 10m, then a T-130-6 core is probably acceptable,
although it will require more turns for the same inductance.
(But if the original design covered down to 80m, you won't need
as many turns if you only need it down to 40m.) So there will
be a bit of readjustment of the number of turns for each tap
point if you are trying to reproduce the original.
The core size required will depend on the power level: for QRP
work you can use a smaller core (again, it may require a change
in the number of turns).
Note that, for
ferrite parts, Amidon originally resold cores from
FairRite, and used their mix numbers (#43, #31). But in the last
few years the appear to have shifted to other suppliers, and the
materials don't have the same characteristics as the original
FairRite types. Digikey, Mouser, and other commercial vendors
sell the FairRite parts under the FairRite part numbers, rather than
under the Amidon numbering scheme. So you would look for a
FairRite
#2631803802 rather than an FT-240-31.