The PDP-11/15 is the OEM version of the PDP-11/20. Its hardware is not completely identical to the -11/20, though; its KC11 CPU is slightly different from the KA11 CPU of the -11/20 (although the KC11 contains mostly the same boards as the KA11).
The KC11 only supports a single interrupt priority level (and thus has only a single interrupt bus grant line on its UNIBUS). It could be upgraded to the 4-level of the KA11 by the optional KF11-A. The KC11 also does not have power fail/restart; this could be added with the KP11-A.
The CPU manual ("pdp11/20-15-r20 processor handbook") claims (pg. 137) that the KC11's KY11-A Programmer's Console front panel has only 16 address lights and switches, unlike the 18 of the KA11, but images of PDP-11/15 (above) show 18 locations for address lights (although it is possible that there are no actual lights behind the two left-most locations).
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Clones: CM 1420