The 915R included in the HAF Stacker 935 is a small case roughly half to a third of the height of the main unit. There is another 915F optional section that is outwardly similar to the 915R aside from the same front panel connectors found on the 925 and the interior mounting for the power supply being at the front of the case rather than the rear. Both share the same mesh top panel and side panels with either three 120mm or two 140mm fan mounting holes. There is a single 5¼" external bay.
The rear of the 915R has an opening for a power supply directly over the I/O opening for miniITX boards. Two riser slots are available for installing cards. Unlike the 925 lower section, these risers are external with the bracket support thumbscrews accessed from outside the case. The top mesh panel of the 915R slides on utilizing the same interlocking rail system that connects the two units together.
The bottom of the 915R shows the interlocking slide rails to connect it to the main 925 unit, along with one medium sized grommeted opening and four small cable pass through holes. Compared to the large openings complete with fan mounting holes in the top of the 925, these in the bottom of the 915R seem paltry. There's very little airflow possible and even the one medium sized opening is barely big enough to pass a CPU water block through, if installing an AIO cooler radiator in this top section for example.
Let's have a look inside the units.