Hi Mark,
The walk through transom referred to as a MKll design began in 1990 I'm pretty certain. The original style was from 1990-1993 where the design of the boat was very similar to its predecessors. After 1993 there were other changes to the design which adapted more of a European design with the beam carried to the aft and a wider stern.

Mark,

Welcome!  Here's a good discussion of the C36 variants: Mk I, Mk "1.5" and Mk II.

All great boats.

https://www.catalina36.org/forum/general-discussion/mark-i-vs-mark-ii

[quote=Fiddler's Green]... One feature we would like our new boat to have is a step-through transom. .... Has anyone had the transom of their 36 modified to open up like the photo attached?
[/quote]
   I don't know if anyone has modified a C36 to put in a step-through transom (would be a major job, although I recall seeing an article once of someone adding a sugar-scoop transom to an older boat), but several of us have modified older C36s (MkI) to add a swim platform. See these earlier threads:
https://www.catalina36.org/forum/technical-discussion/mk-i-transom-angle
(scroll down and scroll down a bit for photos of the completed installations) and https://www.catalina36.org/forum/technical-discussion/adding-swim-platform-catalina-36
   There are plusses and minuses to each model (e.g.: the step-through certainly gives easier access, but in so doing sacrifices a goodly amount of storage in the aft lazarette of the older boats). 

Mark
Have fun at the boat show ,will be going next week. We love the walk thru very nice when having a large group or party easy for everyone to get on board.We live in south sound and have had ou boat for 10 years and have done lots of mods. hope you find a good boat and maybe we will see you on the water

Mark,
I had hand rails made for my pass through on my MK1.5 (1990) I wanted the more classic model when I was doing my search back in 2003. I too wanted a pass through for reasons of getting in and out. Initially though when you pull up in a dingy the ladder was not easy to hold onto getting in and out. I had these rails made so that they went right up to the top of stern rail. I can't tell you how many times we have appreciated this addition. It serves several functions i.e. something to grab when you are approaching in a dingy, able to hold onto when lowering the ladder, additional tie off for visitors, and a huge asset when someone older needs to board. Another note about your comment on storage. I have found that there is plenty of storage on the 36. In fact I haven't taken full advantage yet & I don't use the aft cabin as a garage. I had a Cat 30 with the aft lazerette and I don't miss it.