Had a tiring yet fun time buying a used piano. Thought that some of the info that I have gathered will be useful to other would-be purchasers :

KAWAI Age (Year vs Serial No.)
1950-14200 1967-251660 1975-785568 1983-1372000
1952-15000 1968-303686 1976-844362 1984-1465000
1955-20590 1969-369088 1977-904384 1985-1549000
1958-23200 1970-425121 1978-973490 1986-1627000
1960-26000 1971-488834 1979-1067130 1987-1704000
1964-13230 1972-558216 1980-1126366 1988-1779000
1965-16517 1973-633601 1981-1219355 1989-1850000
1966-20343 1974-718768 1982-1300000 1990-1921000

YAMAHA Age (Year vs Serial No.)
1958-89000 1968-689000 1978-2583000 1988-4492000
1959-103000 1969-809000 1979-2812000 1989-4650000
1960-122000 1970-978000 1980-3000100 1990-4808000
1961-149000 1971-1169000 1981-3261000 1991-4966000
1962-188000 1972-1317000 1982-3464000 1992-5124000
1963-237000 1973-1510000 1983-3646000 1993-5282000
1964-298000 1974-1743000 1984-3832000 1994-5440000
1965-368000 1975-1943000 1985-3988000 1995-5598000
1966-489000 1976-2144000 1986-4157000 1996-5756000
1967-570000 1977-2383000 1987-4335000

Source : http://www.bluebookofpianos.com/

After listening to many models, I was partial towards Kawai. Here are more details on the Kawai's age vs its model name :

Models Dimension (mm) Weight Production
CD300 1560w x 1320h x 670d 260 Kgs. Aug/1983
CD500 1560w x 1320h x 670d 253 Kgs. Aug/1983
CD600 1560w x 1320h x 670d 260 Kgs. Aug/1983
CD700 1560w x 1320h x 670d 259 Kgs Aug/1983
CD800 1560w x 1320h x 670d 260 Kgs. Aug/1983
CL4WO 1460w x 1040h x 530d 165 Kgs Apr/1985
CL4E 1460w x 1040h x 530d 165 Kgs. Aug/1985
CL7WO 1450w x 1080h x 570d 200 Kgs. Apr/1985
CL7C 1450w x 1080h x 570d 200 Kgs. Apr/1985
CL4I 1460w x 1040h x 530d 165 Kgs. Apr/1985
NS10 1530w x 1240h x 590d 215 Kgs. Jun/1984
NS15 1530w x 1250h x 610d 230 Kgs. Jun/1984
NS25 1530w x 1270h x 610d 235 Kgs. Aug/1984
NS35 1560w x 1320h x 670d 255 Kgs. Aug/1984
KL11WI 1460w x 1110h x 600d 200 Kgs. Nov/1982
KL11KF 1460w x 1110h x 620d 200 Kgs. Nov/1982
KL68C 1540w x 1320h x 660d 265 Kgs Jan/1984
KL70W 1540w x 1320h x 660d 260 Kgs. Jan/1984
KL90R(B) 1560w x 1320h x 670d 265 Kgs. Nov/1983
KL76S 1560w x 1320h x 660d 255 Kgs. Nov/1983
KL82W 1560w x 1320h x 670d 255 Kgs. Mar/1984
LK63T 1520w x 1270h x 620d 240 Kgs. Apr/1984
BW52 1530w x 1250h x 610d 230 Kgs. Mar/1985
BW61 1560w x 1320h x 670d 255 Kgs. Mar/1985
CL3 1520w x 1220h x 560d 210 Kgs. Aug/1984
CL4MW 1460w x 1040h x 530d 165 Kgs. Apr/1985
US50 1540w x 1320h x 660d 260 Kgs. Aug/1985
US55 1540w x 1320h x 660d 265 Kgs. Spt/1985
US65 1540w x 1320h x 670d 270 Kgs Sep/1985
US75 1560w x 1320h x 670d 270 Kgs. Sep/1985
US95 1540w x 1320h x 670d 275 Kgs. Sep/1985
BW69 1540w x 1320h x 660d 265 Kgs. Jun/1986
ED50CA 1470w x 1110h x 570d 208 Kgs. Oct/1986
BS10 1530w x 1240h x 600d 215 Kgs. Oct/1987
BS20 1530w x 1250h x 600d 220 Kgs. Apr/1987
BS30 1530w x 1270h x 600d 225 Kgs. Apr/1987
BS40 1560w x 1320h x 670d 250 Kgs. Apr/1987
KL65B 1530w x 1250h x 600d 230 Kgs. Mar/1988
KL72W 1530w x 1270h x 610d 235 Kgs. Mar/1988
US63H 1540w x 1320h x 660d 265 Kgs. Mar/1988
BS20S 1530w x 1250h x 600d 220 Kgs. Apr/1988
ED52S 1470w x 1110h x 570d 208 Kgs Oct/1986
ED48E 1470w x 1110h x 570d 208 Kgs. Oct/1986
KL95R 1560w x 1320h x 660d 254 Kgs. Sep/1990
KL78W 1560w x 1320h x 660d 255 Kgs. Jul/1989
KL75W 1540w x 1320h x 650d 255 Kgs. Jan/1990
KL74K 1560w x 1320h x 650d 245 Kgs. Jan/1990
KL68W 1550w x 1320h x 650d 245 Kgs. Jan/1990
KL64K 1520w x 1250h x 600d 225 Kgs. Sep/1990
KL53K 1530w x 1250h x 600d 220 Kgs. Jul/1989
KL51WI 1460w x 1110h x 590d 200 Kgs. Jul/1989
KL51KF 1640w x 1110h x 600d 200 Kgs. Jul/1989
BS3A 1560w x 1320h x 660d 245 Kgs. Jan/1990
BS2A 1530w x 1250h x 590d 215 Kgs. Jan/1990
US9X 1540w x 1320h x 680d 264 Kgs. Sep/1990
US8X 1560w x 1320h x 660d 259 Kgs. Sep/1990
US6X 1540w x 1320h x 650d 255 Kgs. Jul/1989
CL5E 1460w x 1050h x 530d 166 Kgs. Oct/1990
CL5 1460w x 1050h x 530d 166 Kgs. Sep/1990
BS1A 1520w x 1220h x 590d 217 Kgs. Sep/1990
BS25 1530w x 1250h x 590d 219 Kgs. Mar/1991
US7X 1540w x 1320h x 650d 230 Kgs. Mar/1991
US5X 1490w x 1250h x 590d 232 Kgs. Mar/1991

Source : http://www.yasumaru.com/

I ended up buying a Kawai US63H

Cheers!