BRIDGEPORT MILLING MACHINE "J" HEAD SERIAL NUMBERS. The serial number is on the knee of the machine It can be obscured by the chip shield. Date Start S/N End S/N 1938 BH1 THRU BH39 Round ram 1939 BH40 THRU BH252 1940 BH253 THRU BH656 1941 BH657 THRU BH1549 1942 BH560 THRU BH2943 1943 BH944 THRU BH4105 1944 BH4106 THRU BH4997 1945 BH4998 .