There are various websites obtained by Googling, but nothing as a complete guide as far as I am aware. There are of course two main types, boxlock and sidelock.
Of the sidelocks, the 303E has both the Kerstens and the Underbolts and was (as far as I know) the 'top' model. It has hand detachable sidelocks with a 'hidden' catch. It is also found in a 'delux' form with very fancy chequering edging etc.
The 203E has just the Kerstens (no underbolts) and a different (cheaper?) ejector system. The sidelocks are still hand detachable, but with a Holland & Holland style thumbscrew.
I believe this has now been replaced by a 2003C model which has just the Kerstens and has selectable ejectors.
Current boxlocks are the 2002C - which is sideplated and has the Kerstens - with Anson & Deely style locks with intercepting safety and selective ejectors. The 2001C which is similar without the sidelplates and the 2000C - which is similar to the 2001C, but different (less?) engraving.
There are some details at
http://www.merkel-die-jagd.de/en/produkte/gewehre/bockgewehre-reihe-2000/bockdoppelflinte-2003cI don't have any data on the older guns, but own a 203E and a 303E. They are particularly well balanced and quick handling guns (for o/u guns) and are respected for high grade engineering in the British gun trade, but are poor sellers as used guns and have heavy depreciation from new, which is a shame.
I have heard that the 303E may still be available to 'special order'. AyA's top models (Model 37 and Augusta) are similar, but NOT the same, one having I believe the underbolts, but not the Kerstens, the other having both.
