In the 1990's, two or three other persons and I looked over 40,000-80,000 gallon pots, each with a Turpentine polyembryonic seed in it, to remove off-types and weaklings, and to transplant some of the better extras.
One year genetic testing was done to check on our decision making about the more similar-looking ones, not the grossly obvious ones. The nursery owner and I both got better than 80%.
Often the strongest-looking ones are off-types. Of course, grossly contorted ones and chlorotic ones and strangely colored ones and greasy-leafed ones are usually off-types.
The early strength of mango seedlings is largely determined by embryo (seed) size--- so early vigor is neutral for determining standard (a.k.a. type or "pure") versus off-type Turpentines. Nurserys that prefer the biggest seedlings are "keeping it simple" for their own immediate convenience, but many of these plants will no longer be vigorous when they run out of food stored in the seed.
A "type" (think of "typical") is a clone of the mother tree and does not have a father (pollen grain).
An off-type is produced by sex--- even when a tree has sex between its own flowers, variation is produced.
On the market one can find Nam Doc Mai #4 and others--- which variations presumably are off-types which arose by Nam Doc Mai trees' self-pollinating.