How to mark progression to "senior" in CV

During my career, at one time I was promoted from 'Junior xyz' to 'Senior xyz'. 

That had been a reflection of my progression in the subject matter, whereas my role and responsibilities had remained largely identical. How would I best go about putting that into an MT style CV. 

I do NOT want to mark the entire extent of the xyz position as "Senior", because I was not, AND because the move to 'Senior' i.m.o. shows 'progression'. 

On the other hand, simply adding a new line with a new time-range and a new title, but no additional info to add seems awkward. Since this part is so far in my past, I cannot realy tie a suffiient amount of responsibilties or achievements to either one or the other positions. 

What I have right now is this: 

Jan 2000 - Jan 2001: Senior xyz, ACME - Achieved promotion to "senior" grade in identical role. 
Jan 1999 - Dec 1999: Junior xyz, ACME - Responsible for ... . 
 
 
Another question is: when starting a new role on the first of the month, the timeline looks like the above. Is this sufficiently un-ambiguous, or does it leave a hiring manager thinking that I want to dust over 8 weeks of undocumented time? 
 
THANKS!