Mind you, the older Seamasters 2531 were alot better than the modern versions that our trusted dealers carry. I beleive all of them carry the model with the 2824 movement. There are similiarities and there are differences. The 2824 movement just uses the date function whereas the 2836 uses the day and date function. Other differences are the following (thanks to Randy):
2824 is much thinner in profile than the 2836. They're both 11.5"', but the 2836 is 5.05mm in height vs. 4.6mm for the 2824.
The 2824 is closer related to the 2892, which is thinner yet- 11.5"' x 3.6mm.
Hope that helps.