2,17α-dimethyl-5α-androsta-1-en-17β-ol-3-one, or 2,17a-Dimethyl-17b-hydroxy-5a-androst-1-en-3-one
660:90-170 vs. methyltestosterone by oral administration. 
Methyl stenbolone, methyl sten, 17a-methyl-stenbolone, m-sten, ultradrol
In 1966, researchers at Searle Laboratories set about methodically testing the myotrophic (anabolic) and androgenic effects of a series of A-ring modified androstane derivatives.  The compounds they explored reads like a who’s who of designer steroids.
Methyl-1-testosterone (M1T), desoxymethyltestosterone (phera), 17a-methyl-1-androstenediol (Alpha One), and a variety of other 1- and 2-dehydro compounds were explored for activity.
The researchers proudly announced that “Even the least active compound in Table 6 possessed a higher relative myotrophic potency than previously has been obtained with several clinically interesting compounds which have been studied under identical conditions, i.e. oxymetholone, oxandrolone, stanozolol, and methandrostenolone.” (anadrol, anavar, winstrol, and dianabol). 
IIe = methyltestosterone
IIa = methyl-1-testosterone
IVd = methyl stenbolone
IIf = alpha one (17a-methyl-1-androstenediol)
IIIa = phera (desoxymethyltestosterone)
As you can see from the table above, methyl sten has somewhere between 2/3 and 3/4 the anabolic activity of methyl-1-testosterone, and a similar A:A ratio (by oral administration to castrated rats).
It can also be found in an earlier paper by two of the same authors,  however at the time it was only studied for activity by intramuscular injection, so the figures it quotes are irrelevant for our purposes. It does, however, give a recipe for producing the compound from superdrol.
Methylsten™ is listed as one of the ingredients in a proprietary blend in a now-discontinued product called Mass Tabs by IDS, though testing has shown that this product (at least the later, bottled batches) in fact contained superdrol. 
Structure and Function:
Structurally resembling the bastard child of M1T and superdrol, methyl sten is a DHT-derivative that is dimethylated at C-2 and C-17 (like superdrol) and has a 1-ene (like methyl-1-test).
It’s important to note that while methyl stenbolone is dimethylated at C-2 and C-17 like superdrol, the spatial configuration is different due to the presence of a delta-1 double bond (the C-2 methyl group is therefore planar). This means that methyl sten is a 2,17a-dimethyl rather than a 2a,17a-dimethyl compound.
A more recent (2009) paper on the effects of structural modifications to steroids concluded that the addition of a 2-methyl function to a 1-ene steroid had little effect on the relative potency of the compound. 
You’ll note however that this is view is taken virtually word for word from the 1961 study that only examined the activity of the compounds by IM injection and is therefore questionable when discussing their oral activity. 
Since it’s DHT-derived, aromatisation is impossible. 5a-reduction is also impossible, since it’s already 5a-reduced. The 17a-methyl group greatly increases the bioavailability of the compound by oral administration.
The combination of delta 1-dehydrogenation and 2-methylation is likely to make the A-ring very resistant to metabolism. 3z-,16z-, and 18-hydroxylated metabolites are likely to be the only ones detectable after administration, other than the unchanged compound. 
It is a strong oral steroid in the vein of pheraplex, superdrol, and M1T. It should be an excellent “bulking” compound at an appropriate dosage.
It is inevitable that the compound will display some degree of hepatotoxicity. This is discussed in some detail on the manufacturer’s blog, which is recommended reading. 
The only confirmed product on the market to contain this compound comes in 4mg caps and recommends dosing at two caps per day (8mg), and not to exceed three caps per day (12mg). In practice some users are dosing up to 24mg.