Stenbolone actually closely resembles the steroid methenolone (Primobolan). The only difference is that instead of a 1-methyl group, it has a 2-methyl group. The same difference that separates drostanolone (Masteron) and mesterolone (Proviron). In characteristics this changes very little, one can assume stenbolone to have roughly the same effect as methenolone. It's a 5-alpha-structure, a 5-alpha reduced version of boldenone. That means it doesn't aromatize to estrogen and does not cause problems with bloating through water retention, or gynocomastia (growth of breast tissue in men). That took care of the first problem. But at the same time the 1,2-double bond in its structure made it less androgenic than one would expect from a 5-alpha-reduced steroid. As with boldenone, which is only half as androgenic as testosterone, Stenbolone is only half as androgenic as Dihydrotestosterone, the 5-alpha-reduced version of testosterone. So despite the fact that it did not cause estrogenic problems, it didn't really cause androgenic problems to a great extent either. So a user would worry less about hair loss, acne, prostate hypertrophy and deepening of voice than he would with say, testosterone.
So what really is the difference between stenbolone and methenolone (Primobolan)? Well, there really isn't one. The 1-methyl group serves a function to increase oral activity. But since there is no oral stenbolone, that point is irrelevant. The 1-methyl group serves no real purpose to the injectable form of methenolone. So they are the same. I would assume the addition of a 2-methyl group on stenbolone was the same as that of drostanolone, to protect the 3-keto group and improve stability and androgenic binding. But here too the alteration is fairly useless. The reason methenolone and stenbolone have reduced androgenic activity is the 1,2-double bond, but that same double bond also keeps it from being readily deactivated by the 3a-hydroxysteroid dehydrogenase enzyme, as the case is with Dihydrotestosterone (drostanolone is 2-methyl-DHT). So the need for the 2-methyl alteration is minimal at best.