SS-109 is Fabrique Nationale's (FN's) name for their 61.5 grain bullet with the steel penetrator in the nose and what they call rounds loaded with this bullet.
No mention of the intent to be heavier with no size increase (which is not possible trading lead for steel) but to add a penetrator.
You are correct, sir.
I was mis-remembering.
The 62gr round is slightly longer, which is why the 1/7 twist rate was derived for the M-16A2. The bullet is slightly longer, but deeper set into the case so that OAL will fit in the standard magazines.
The "penetrator" has marginal effects, which is why the M855 round isn't considered armor piercing by either the military or even US code.