SLR is a tape format other than DAT:
https://en.wikipedia.org/wiki/Scalable_Linear_Recording
What's the back of this thing look like?
Based on what's in here, it almost seems like this is a rather early D2D2T type of arrangement to get around the fact that SLR looks like it would have been painfully slow. Based on some really rough napkin math, it would've taken 180 minutes to write a tape. (three hours) Inside of working hours, that's 180 minutes you can spend doing something else. Outside of working hours, you can set a machine to write a backup to the hard disk, then let the tape drive noodle on writing and verification for however many hours it wants.
In terms of salvage, consider leaving it together, especially if there are any identifying labels that might be used to tie this together as a specific product, because as far as I can tell this is completely unique for the time. This kind of build isn't uncommon today, and it's used for exactly what I described, compiling a bunch of info to write to tape before bothering to deal with the tape drive. D2D2T systems are often larger, SANs or NASes that servers back up to and then write to tape all at once, instead of each server having a tape drive, so I'd be
intensely curious to see any info we could find on how this thing was marketed.
Otherwise, the most valuable thing from it is probably that 4.5-gig hard disk, provided it works.
The other possibility is that this was some kind of network appliance, or something else like a voicemail server with a built-in backup. Iinfo on labeling and pictures of the whole unit as assembled including other connectors could help make this judgement.