If you do a search of 'Singer 15 clones" all kinds of stuff comes up. I printed out an article that I thought was really interesting on the subject of the 'clones' when I got my first 'clone.'
This article was written in 2006 by Ed Lamoureux and I found it explained the Japanese 'clones' very well. To quote from the article, "Shortly after WWII, Japan, with money from the US, manufactured a large number of sewing machines. The most common of these were based on the Singer Model 15, but there are also Singer 99 clones. Indeed many of these machines are practically indistinguishable from a Singer and use parts that are interchangeable. Often they were given American sounding names to appeal to the overseas market. Over 5000 different "brands" have been identified, manufactured by 15 or so companies. Unfortunately, records from these manufactures don't exist, so it is impossible to identify them further. Generally speaking, a machine will somewhere state "made in Japan" or have a "JA" stamped into the bottom of the machine. To further complicate matters, large retailers would purchase machines and have the company name on them: RH Macy, Gimballs, etc. Any retailer so inclined could have Sewing Machines made just for him or her (Sears Kenmore, Wards Signature). I've tracked Department Stores, machines with automobile names, female names, patriotic names, etc. The post war machines are generally well made, often quieter and smoother running than the Singers they were copied from. Japan also gave us many of our colored sewing machines, examples have been found in metallic blue, green, pink, yellow, and I have a Fire Engine Red one!~ In terms of collectability, don't be fooled by a claim of 'an extremely rare" Mitsubishi, Ford, Saxon, Stitch Queen, etc., the same machine could have dozens of different names. If it says Singer on the machine, it probably is. If it doesn't, it's not. The Japanese machines have not caught on with collectors (even the ones made in occupied Japan) and as a result retain very little value. They can be found at Thrift Stores for $10 - $20 and at local auctions for under $5. "
Anyway, since this article was written in 2006, I think very little has changed. The 15 'clones' are great machines. Whether they are in fact actual clones or not they have been called 'clones' for a very long time.