I don't know exactly when Chongqing began early production, Gadge, but the project is dated 2002 in the Briggs history statement:
http://www4.briggsandstratton.com/miscpdfs/RNT/history%20of%20briggs_MS8751.pdf

Here is the list of Briggs locations large enough to be called "facilities" and therefore have plant numbers, as it stood in 2011:
[Linked Image]

That is taken from Page 7 of 32 of a published conformity document called "Briggs & Stratton 856 Ship Notice/Manifest Version 004010", and is headed "B&S facilities and their associated plant numbers are as follow."

As you can see, 57 is not a Briggs plant, so your engine does not have a valid plant number in its Code. This is the phenomenon I was talking about when I suggested that the engines with non-existent plant numbers in their Codes, may come from Chongqing. All such numbers I've heard of so far, are in the 50s. Note that most of Briggs' older plants, all of which have numbers lower than the 20s, have been closed and do not appear on this list. There are also a couple which appear on the list but have been closed since 2011.

All this may mean that you have a very early Chongqing engine, from before the plant's opening ceremony. Alternatively, my whole hypothesis may be wrong - but AFAIK, no-one has ever reported an engine carrying the Chongqing plant code "28". It may only be used on engines sold within China. The reality is that no small side valve engines have been made anywhere but Chongqing in approximately ten years now, and no side valve engines at all, including large ones, have been made anywhere but Chongqing in the past couple of years. All of the Chongqing engines that have been reported to me have Plant Code numbers in the 50s, and those Code numbers do not match the Plant Code numbers of any actual plants.