I believe that it is called Leica APO-ELMARIT-R 180mm ƒ/2.8. This version, the final one, started to be produced in 1998.
It seems to itself have had 2 versions (I: No. 11273 & II: No. 11357, from yr. 2004~) that are identical optically but differ by their compatibility with Leica Tele-converters.
The f/2.8 APO is a very fine lens in most respects. The aperture blades are a little on the short side so at apertures between f/2.8 and f/5.6 (approx) out-of-focus highlights will take on a 'ninja-star' shape - or at least my copy was like this. The focussing ring has very little damping, which is a minor issue, and IMHO it would have been great to have had a built-in rotating tripod collar similar to the 280 f/4 APO-Telyt-R's collar. The collar and a little more damping on the focussing ring and the lens would be just about perfect. Really great lens as it is too.
The modification to allow use of the 1.4x APO-Extender-R involves a slight optical change (moves the rear element forward).
The 180mm f/4 Elmar's primary virtues are compact size and light weight. It's optical performance is nothing to get excited about.
The 180mm f/3.4 APO IMHO is a great bargain, nearly as good optically as the f/2.8 APO. It doesn't focus as close and is said to be optimized for infinity (originally designed for the US Navy to photograph Soviet 'fishing trawlers'). I'd look for the later version that uses E60 filters, or check with Leica to see if they still offer the conversion from Series 7.5 to E60.
The earliest non-APO f/2.8 (has built-in non-rotating tripod mount) is often described as a 'brick', referring to its weight. The later version non-APO f/2.8 is lighter but is somewhat out-performed by Nikon's 180mm f/2.8 ED and by a greater margin the Leica APO lenses.
No personal experience with the Summicron; supposedly the optical performance is very similar to the APO Elmarit. The Summicron is too big & heavy for my uses so my first choice would be the f/2.8 APO-Elmarit, followed by the f/3.4 APO-Telyt.
