MM106a hardiness

I have ben searching the internet seeking info about the hardiness of MM106a.
I am not seeing any hard data about this rootstock in particular.
Does anyone know or can point me to a website that discusses the MM106a hardiness?
I have many trees on MM111 which are doing quite well here in zone 4a (Canada). However, I
am thinking of getting some trees from a nursery that have the varieties I want only on MM106a.
Thanks.

Peter Drevniok
Maxwell Mountain Orchards
Wakefield, Quebec
zone 4a

First time I hear about MM.106a - that does the "a" stand for? I think there is a MM.106 emla variant - could it be that?
As of standard MM.106, the few trees I have on that rootstock don't do very well in my orchard. Lack of vigor and lack of productivity... I think they like it better on richer soil, and maybe in not so cold climate...
Claude

Jolicoeur Orchard
Zone 4 in Quebec

Like Claude, I am not sure what the "a" is for, though it usually means a "cleaned up" version of the original, as Claude suggested. In any case, 106 has been touted as a great rootstock if you want trees within M7 and MM111 sizes. It's downfall is susceptibility to collar rot and low chill requirements. Meaning, don't plant it in replant or wet sites and it likes to "wake up" earlier int he year, pushing early tree growth and increasing the chances of cold damage. It is a very nice semi-standard tree when planted on well drained soils in milder climates.

Mike Biltonen, Know Your Roots
Zone 5b in New York