Chinese famille verte porcelain vase

[wilfychan]

I recently purchased this beautiful famille verte vase.  It has an underglaze blue seal mark on the side.  Can you please help identifying the mark? 

[peterp]

The mark says: Yawan Zhencang (?????, not a mark per se, rather using the seal type characters instead of handwriting.
The decoration looks as if it were Guangxu, the neck shape is less common, however, in this period.