Thomas & Brian,
I believe that in earlier years the Sekai 4000 was mass produced of Tange #1 tubing, perhaps by Yamaguchi, while the Sekai 5000 was a handmade superlight frame made by a guy named S. Higashigawa:
http://www.ece.ubc.ca/
I don't know of any database of serial numbers for SEKAI. If your bike has some original paint you can estimate the time period from the badging:
HEAD BADGE
Swirly Blue Badge (pre-78)
http://www.ece.ubc.ca/
Hexagonal Multicolor Badge (78-onwards)
http://www.ece.ubc.ca/
DOWNTUBE
SEKAI(big, alone) Downtube, Model name (toptube), 73-76
http://www.ece.ubc.ca/
SEKAI(big) + Model number + Model name(stacked) Downtube, 77-79(?)
http://www.ece.ubc.ca/
SEKAI(big) + Model number(big) Downtube, 1980(?)-onwards
http://www.ece.ubc.ca/
TANGE DECAL
CHAMPION (written horizontally), 73-78
http://www.ece.ubc.ca/
CHAMPION (written diagaonally, like 531 in reynolds), 79-onwards?
http://www.ece.ubc.ca/
On the 4000 (I have 2x), I have seen long-point italianesque chromed lugs throughout the 70's, and prugnat lugs often unchromed (later, maybe 1980+). The fork is light mass-produced Tange fork with stamped and welded crown so it is normal for the fork dropouts (Tange TF) to mismatch the Shimano rear dropouts (Shimano SF). In the 1970's both the 4000 and 5000 models were offered as frame-only. There would be no standard build for a 1970's frameset-only offering.
The 4000 model was a reasonably lightweight frameset, see the comparison table of weights below, SEKAI 4000 @ 24 inches weighs 2934 grams with fork crown race installed, about as light as my early 531 Raleigh-Ilkeston Team bike (but with no BB routing or bottle braze ons).
http://www.ece.ubc.ca/
- Don Gillies San Diego, CA
P.S. Also, greetings and salutations to Thomas Dockery of Champaign Cycle (in the hometown of the University of Illinois) - that was and still is a monument to midwestern cycling, and "Got me addicted" to 10-speed bikes at age 10, in 1972, by giving me my first Raleigh Catalog. I left town for college in 1980, so our paths probably did not overlap, but we are probably the same age.