Kath, what model is your BL? I used to sell BL's way back then. The older Proline models are pictured in the book, Know Your Babylock from 1990. Almost all sergers pictured and featured are Proline models and had more TOL new features at that time because they featured easier threading loopers, differential feed, one turn rotation of the tension dial instead of multiple rotations. Differential Feed was the new added feature at that time and added to these ProLine models where older but similar looking models lacked the DF and were not ProLine. These ProLine were enclosed units that never needed oiling by the owner. All of these models were distinguished by number but looked much the same but maybe had a different colored stripe on them and all were the sergers that were developed before the newer plastic Pro Line models and before the first model with BL's patented air threading system. These older models still used the round shank industrial type needles.
The plastic Pro line models that came about afterwards are not the same sergers. i saw one on ebay once and it had plastic casing and not at all the same serger and I don't know where it was made.
The 4738 is one of the sergers featured in the Babylock book and it was before the indtroduction of the air threading. These Early Pro Line models were made to specs in the same factory in Japan.
I purchased mine at the time with an employee discount and the new air threading BL just hit our store. I liked my particular model over some of the others because it was a 4 thread with DF and could also do a chain stitch beside the serged over edge and also a single chain stitch, besides the regular 2, 4 and 3 thread overlocks. After many years of use the serger was still in top condition and when I traded it in for a new coverlock my dealer said they sure don't make them like they used to and priced it for sale for bascially what I paid for it with my employee discount years ago.