If its an 81 it has the incorrect seat tube decal
Theres been a fair amount of research recently about the Ripper SN's and many now think the serial numbers are actually date related. We all know that 79 Rippers were the only ones not to have a SN and the 1st SN came out in late 79 (Nov). From then on 80-84 it's been hard to tell the exact year from the SN, you can tell the year if it has the correct decals fitted.
I've been told by the OM that the serial numbers were indeed random, however heres what folk are talking about now.
1978 (JUs, DB-III's). No SN's. A couple RIs with 1xxxx's exist.
1979 (PK, DB-IVs'). No SN till later in the year, started with 2xxxx.
1980 3xxx There are plenty of 79's with PK3 SN's
1981 4xxx
1982 5xxx
1983 6xxx
1984 7xxx
1985 SE in house now. New back end, and, new SN style.
so in short, yours is probably an 83