Question re valve types?

[Zylog]

Is anyone able to tell me what difference if any, there is between a 12AX7M and a 12AX7A valve.

Thanks

 

[Lord]

Following a quick Google, I think that 12AX7M is a 'Groove Tubes' Mullard reissue, hence the M.

 

[Zylog]

Thank you.