I think you're thinking of the Science fair microcomputer trainer:
which had a TMS1100 as the processor (if not there is the digital computer kit on the previous catalog page but I don't think that matches your description).
This page has some more details
The manual says nothing more than it can print 12 characters per second and it uses BUSY pin.
Even if you have connected BUSY output to CTS input, and turned RTS/CTS handshaking on, there is still a possibility that one or two extra bytes are sent out on the data pin, because the serial string write happens as a single large block, and also the USB packets ...
Page 3 of this 1983 Radio Shack catalog shows the Data Drawer in two different sizes, "printout" which holds 14-7/8×11-inch 132-column printer paper, and "letter" which holds up to 8-1/2×14-inch legal size paper. There are also inserts for cassettes and printwheels.
Following up on @snips-n-snails.
The use case for this is to organize printouts for reference. A simple example is that if someone wanted the ID of a vendor or customer, it was not untoward at the time to not look it up on the computer, but, rather, pull out a printout and look it up there. This organizes printout.
You can easily envision a printout for the ...
Going thru my junk box, I found the remains of a computer trainer that I think I got at Radio Shack. I attempted to hack it, but soon lost interest. 30+ years later I am ready to play around with it again. It used a TI MP1312 CPU. I think this may be the trainer in question. I cannot find any info on CPU.