3- speeder is correct that the J7 would be seventh month ( July ) 1942. July is the month that the frame would have been manufactured. After a 3 month freeze on the sale, but not production of bicycles ( late March through June 1942 ), the war production board (WPB) allowed the sale of bicycles with many restrictions to resume in early July 1942. Westfield was one of two manufacturers, the other was Huffman, that were allowed after Sept., 2, 1942 to cont. producing bicycles. The other bicycle manufacturers could by special appeal be granted authority to produce limited quantities. Your serial number W105355 would indicate that this particular bike was completed most likely in the last quarter of 1942. My guess would be Nov. or Dec. One of the restrictions placed on the bicycle manufacturers in Sept., 1942 was that any bikes produced could not show indication of the maker. That is why your bike is missing the Columbia head badge. It was never attached at the factory. Some frames that were produced prior to Sept., 1942 but not used until after that date will still show the two tiny holes where the badge would have been attached. Most frames made after Sept., 1942 would not have the badge attachment holes. Westfield and Huffman were allowed to use the letters W ( Westfield ) or H ( Huffman ) as the prefix letter to the serial number. Evidently, military model Westfield and Huffman made bicycles were still allowed head badges on those bikes, which were however, painted over in OD from the factory. The chain guard is not original to this bike. There would not have been a chain guard on civilian bikes from the factory during the period that your bike was produced.
Regards,
Bill