Boyar schultz 612 age identification

I am wondering if anyone out there may have a way to identify the age of the 612's.....I have 2 units that I know are quite old, both units are belt drive units and it would be nice to know how old they are. The serial numbers for the units are as follows:

9618

13919

I'm sure they are very old.

The 4 digit unit has a blower on the back of the cabinet that provides suction on the base cabinet and there is a hose that attaches to the back of the cabinet and to a scoop on the end of the table. It is need of rewiring as the insulation is brittle and missing in several areas.

The 5 digit unit is somewhat "newer" and just has a storage cabinet. This unit was just powered up and has the noisy spindle bearings.....I am planning to do a tear down and clean relubricate or replace the bearings using RLamparters excellent posting....

Thanks to all for any help in this area

Model/Size: 6" x 12"
1951 ............................................................... 5500
1952 ............................................................... 6500
1953 ............................................................... 7500
1954 ............................................................... 8000
1955 ............................................................... 8500
1956 ............................................................... 9500
1957 ............................................................. 10000
1958 ............................................................. 10500
1959 ............................................................. 11000
1960 ............................................................. 12000
1961 ............................................................. 12500
1963 ............................................................. 14000
1964 ............................................................. 14500
1965 ............................................................. 15500
1966 ............................................................. 16500
1967 ............................................................. 18500
1968 ............................................................. 19500
1969 ............................................................. 20500
1970 ............................................................. 21268

Thanks to vc3006 your post was invaluable.......