It's configuration is as follows:
This machine started out it's life as a VAXserver 3500, and it still says so on the front. As I obtained the inwards of a MicroVAX 3900 I replaced some of the boards, including the CPU which means that machine went from a KA650-B based VAXserver 3500 to a KA655-A based MicroVAX 3800. The difference between a MicroVAX 3600/3900 and a 3500/3800 is in their cabinets and expandability. The MicroVAX 3500/3800 are in BA215 enclosures with space for up to two RA70 series drives and a TK50/TK70 tape drive, while the physically bigger, 19" rack mounted, 3600/3900 can accomodate e.g. two RA90 series in the rack.
As opposed to the MicroVAX 3300/3400 (and later MicroVAX 4000) machines these machines have neither NI nor DSSI support on the CPU board. Therefore the (slower) QBUS based expansions for these capabilities must be used. Normally these machines uses SDI disks instead of DSSI hanging on a KDA50 controller built out of two, connected, circuit boards.