Good question. I've had a couple 435A's over the years and had seen but never had 435's, and had
assumed that the difference was the interchangeable scales on the 'A'. However, I recently obtained
another 'A' model, and it has the non-interchangeable scale. At first I thought someone had swapped
knobs with a non-working 435 until I realized that it has the marker where it belongs, just above the
outside circumference of the scale. The two 435a's are identical except one is a 1415A prefix with the
old style range indicator and the other is a 1629A prefix with the interchangeable scales. Apparently
the difference is other than what I've been able to detect without getting inside. Maybe when I get more
curious I'll do an inside examination.