This is an external SCSI hard drive housing that connects to the Amiga's parallel port. It is not clear if this is a version 1 or 2 - it is assumed to be version 1.
It is made by Expansion Systems (possibly also known as or contracted to/by M.A.S.T. - Memory And Storage Technology). Not a great deal is known about this unit, but it's clear from the sticker on the front that this model was the 30mb version.
It has an internal 50 pin SCSI header for the hard drive. It has a power supply connection and a disable switch on the back, and has a passthrough for the parallel port. It also has an external 25 pin SCSI connection - it is not clear if this a SCSI passthrough or an input to allow some other SCSI controller to access the hard drive inside directly. The housing is metal. On the circuit board is printed "Amiga 500 SCSI" - it has been tested to work on the Amiga 1000, so may work on other Amiga models depending on software compatibility