Meyers Lemon was introduced to USA from Asia and later found to be a symptomless carrier of the citrus virus Tristeza. This does not affect Meyers Lemon in any way, but can allow the disease to be passed on by aphids to commercial citrus varieties which are affected. The virus can be eliminated by various laboratory procedures such as heat treatment and shoot-tip grafting. Clean Meyers Lemon was propagated on a large scale in, I think, the 1960's and all old line Meyers should have been replaced with the new 'Improved Meyers Lemon'. Certainly, no old Meyers should now be sold in the USA.
There is no difference between 'old' and 'improved' Meyers (apart from the lack of Tristeza). They are the same variety.

I agree with citrange.

Thanks!! I never knew why it was called "imporved'. This is really good to know.

DL