In general you can split all zig zag sewing machines in two groups. The no. 1 type is when the hook pass behind the needle, and the needle is threaded from the front. The no. 2 type is when the hook pass the needle to the side and the needle is threaded from the side.
I think all Singers and Pfaffs are made as the no. 1 type except the Singer 237 mainly produced by the Necchi factory in Italy. The no. 1 type has been most successful and almost all new machines are of this type. But I think the no. 2 type have got some advantages, and many of these Japanese produced machines are made this way. They should be better to handle heavier work, because they are less sensitive to at bit forward and backwards forces on the needle from the fabric.
I made this video explaining at bit of this type 2 regarding Singer 237:
https://youtu.be/8TONKVFi1fg
I noticed a youtube channel, that have made a video description of many of these Japanese badged machines, because they restore them and sell them. I have no interest in this shop, but I think their many videos are a very good source of information about these machines:
https://www.youtube.com/user/tonysmo...able_polymer=1
In most cases, you will be able to see what kind of the two types the machine is, by looking at the direction of the needle plate and to the left you got the sliding plate to access the bobbin.