Model 18 and 20 T-case differences

I have the chance to add some cast iron to my "pile-o-parts" inventory and am slightly confused about the fine point differences between the model 18 vs 20.
The donors pile-o-parts has loose 18 parts mixed with 20 parts.
How can you visually id a stripped model 18 case vs a model 20 case. if both are big hole, 1 1/4" int. shaft cases?
I think I read here that 18 gears will fit into a 20 case but not vice versa?
I think I read that both had a fill plug on the side, but not all 20 cases had one?
Also, later 18s came with a single stick shifter, and werent all of the 20s single stickers?
What exactly will interchange? can you convert a 20 bare case to dual stick, with all the internals from an 18? Its a little confusing when I cant see them side by side.

 

You can't use 18 parts on a 20 to convert to twin stick unless you convert the 20 case to an 18 transfer case. Doesn't work.
You can put the 20 gears in an 18 case and vice verse and can use a 20 case for an 18 but cannot turn an 18 case in a 20 because of the shift rod length of the 20.
To visually id a bare 18 case from a 20, look at the back of the case where the relief for the shift rails is at. The 18 has one relief that is cast as part of the case. The 20 has two caps that press in that are the reliefs. There were some rare model 20's used from about '67 to '69 in IH Scouts that were twin stick models. Other than that, all 20's were single stick from the factory, but there are many variations in linkage depending on model of vehicle, transmission used, etc. Some late model 18's were single stick, primarily behind V-6's, but some have reported having one behind an F-head. Most F-head and L-head Jeeps had a twin stick 18. Without showing pics of the different types of linkage, No way could I describe all the differences over the years, just too many. Hope this was somewhat helpful and not too confusing. Nickmil.

 

Easiest way to tell is to look at the rear output of the transfer case. It will be offset to the right, in line with the front drive output, or it will be centered, not in line with the front drive output.

18 = aligned with front drive

20 = not aligned with front drive.

 

Kamel, this doesn't work, as he mentioned these cases are stripped and both the 18 and 20 cases are nearly identical to each other when stripped especially to someone who doesn't know the little tips like Nick mentioned to tell them apart.