On the 850TC one-piece alloy oil pan, there were 3 round mounting bosses on the exhaust side of the pan. Looking directly at the left (exhaust) side of the oil pan, the upper right mounting boss (located just about below the fuel pump) was used to connect the muffler mounting bracket. The center/bottom mounting boss was usually used for fitting an oil temp sensor. I've never seen anything "officially" used in the upper left mounting boss (closest to the flywheel).
On the 1000TC two-piece alloy oil pan, I've seen the same set-up as mentioned above, though some pans have only 2 mounting bosses instead of 3. In this case, the upper left mounting boss was often used as a primary (or secondary) muffler-bracket mounting point, especially with some of the period megaphone-style exhaust systems.
Some of the 750-type alloy oil pans had 2 mounting bosses on the exhaust side, similar to on the 850TC pan, but without the third center/bottom mounting boss. There were various aftermarket exhausts that featured 2 side bracket mounting points, similar to those used on the standard Fiat 600D exhaust.
-JS.