If you look on Google or in textbooks there appear to be many different versions of the model. In fact it may help students to have a look at some of the different versions to enhance their understanding of what it is all about.
This one has a rehearsal loop for STM, as many of them do. This one shows information being forgotten – others show retrieval from STM. This one includes quite a few other details. Have a look at the others. They tell you something about the nature of knowledge.
And this is the one from Atkinson and Shiffrin’s original article.
You may be interested in their comments:
The second basic component of our system is the short-term store. This store may be regarded as the subject’s “working memory.” Informa- tion entering the short-term store is assumed to decay and disappear completely, but the time required for the information to be lost is considerably longer than for the sensory register. The character of the informationintheshort-termstoredoesnot dependnecessarilyuponthe form of the sensory input. For example, a word presented visually may be encoded from the visual sensory register into an auditory short-term store . (Atkinson and Shiffrin, 1968, page 93).
You can download the article here, or send me an email and I’ll give you a copy.