I just got my second aero ektar, i noticed this one was different. It's lighter, and has a non removable filter mount. So i checked up the serial number, and it dates to -42. I know the patent for this lens was made -41 so this must be very early model. It's serial number EA1006. What is the lowest number you have seen? Was there optical changes to this lens along the way? At least it seems to have some sort of ring after aperture ring, like it might go faster than 2.5 without it, or maybe it limits the image circle. I don't remember seeing this in my last aero, which i sold.

And yeah, it's the common 7" model.