Looking for info on National Semiconductor RAM board (VAX 11/730)

I am looking for any information on a National Semiconductor RAM board that I think goes in a VAX 11/730. My 11/730 (which admittedly I have not run) has 2 DEC MS730 boards and 2 of these NatSemi boards, I found another one while unpacking stuff today. It's a hex height DEC-type board with the ejector handles. Connections to fingers on A and B as you might expect, also the grant continuity strapping on C and D, grounds on D (only) and a couple of signals on C (I've not figured out what yet). The only identification I can find on it is in the etch : PWB 551109464-002 B PWA 980109464 (I assume the first is the part number for the bare PCB, the second for the stuffed board) There seems to be one RAM too many!. The main RAM array is 4 rows each of 32 4164-like DRAMs. There is a second block of 28 such DRAMs, presumably 7 bits for each row of the main array to provide the ECC bits. And then a further single RAM off to one side. The DEC MS730 doesn't seem to have this, and I wonder what on earth it is for. Rest of the board is TTL drivers, etc. There are 2 LEDs, one green, one yellow. And a momentary pushbutton switch. I have no idea of the functions of those. Does anyone recognise this board? A related point, I am thinking of removing the TSU05 from my 11/730 and putting an expansion box in place of the tape drive in the rack. In which case I can install this memory board. Is 5MBytes (the maximum an 11/730 can take) worth having over 4MBytes? -tony