Brophy only covers to '88. The run-of-the-mill 39As were never checkered to my knowledge, only special-edition and commemorative models had fancy wood and checkering. The most common ones you see are the Annie Oakley models and "1897"s. And plastic sights on a 39 are just wrong. But at least you SAW a new one.
Barrel diameter varies over the years on 39s, wouldn't keep me from picking up a good specimen if it had an attractive price even if it was skinnier than some other year's production.
Tall Grass linked you to the mystery of the 39A barrel-they're all over the place. I forgot to mention in that thread that my 2008 had a flat target style crown as opposed to the rounded recessed crown on most Marlins. My '08 rifle is very accurate, but the '47 comes very close.
I started the thread mentioned above. It looks like the larger diameters started about 1978 and continue to the present. I would add that in 1988 the crossbolt saftey and rebounding hammer were added and I would stay away from them. Find a nice older one they are out there. I bought a 1980 at a local auction that someone replaced the stocks with an original checkered 39 stock set. It is really neat.