MF-13 is an extension of a blend that was developed at the same time as the T52. About a year ago there was an MF-4 blend... well we are up to #13 now... a whole lotta adjusting goes on.. that is the fun of blending.
And to answer the question: There are about a dozen or so test blends floating around at all times. Most are works in progress, most will never be put into regular production, but some of the best ones we share with friends and at cigar events or during Cigar Safari.
For example, I am doing a special private LP tasting for Top Shelf in March and I will be sharing not only the MF-13, but two other LP blends that no one has tried...
BR,
Steve
Pres. Drew Estate