UNIBUS RAM with Parity vs. None

(cross posted to MARCH list) Question about UNIBUS RAM with Parity vs. None I assembled a pdp 11/05 in a BK11-K box with a sticky 5 light - I can load an address, but the 5 light will erroneously (often but not always) light when I examine. Once 5 is turned on it stays on, even if the toggle switch is down. Once this occurs I cannot deposit nor can I examine contents of RAM. I intend to test the front panel to verify that toggle 5 is ok electrically, this is a to-do. The power checks out including DC LO AC LO. I transplanted the CPU cards into another system to test and they work also. Aside from the console itself, it must be a RAM problem. The system has 8K Core G235 (xy drive module) H217D (memory stack) G114 (sense / inhibit) M8293 (memory control module) My question - I have read, and to the best of my understanding believe I can swap an H217D with an H217C in this box. Anyone disagree, and if so why did DEC want their 11/05 S to use "D" model core (D=parity) and not C (C=no parity)? The PDP 11/40 I have uses C. -- Bill