Thanks to some persistent fact finding and experimenting it looks like Steve Lilley has found the solution which was hiding in the Wiki all along in https://github.com/fritzing/fritzing-app/wiki/2.1-Part-file-format under the heading Hybrid connectors. Thanks Steve for all your hard work
I checked the link you mention and all the images that should be included in the Blog are not visible. Do you know who can fix that? Thank you.
The website software was replaced awhile ago. It seems that the the uploaded images for the blog are either missing, or located in a different path. Another place to start for information is the creating custom parts tutorials learning page. Another source is the tutorials - guides category here on the forum.
@KjellM is probably the only one with a chance to fix it.
Referenced page : http://blog.fritzing.org/2012/10/09/new-parts-editor-released/
and embedded image link sample: https://fritzing.org/media/uploads/blog/parts_editor_help/start.png
In addition you may want to look at these two sets of tutorials (for the Parts Editor @Old_Grey 's video tutorials are your best bet, I don’t tend to use it.) Both these tutorials apply to the current version, many others around are for earlier (sometimes pre Parts Editor) versions.
Also feel free to post questions here (preferably including the .fzpz file for parts or the .fzz file for a sketch you are having trouble with) as parts creation is fairly difficult to get right until you get experienced.
I repaired the links to the images in the blog post. Note that the post is about the old parts editor (before 2015/2016 I think), but some elements look similar.