Yep, a Chenghua Nianzao mark! Actually, the only thing that could be casting any doubt is the stroke just below the "nian" character...seems to be superfluous. But, anyway, overall this mark is executed in a sloppy manner like many others, in view to calligraphy. If it is from the Wanli wreck, then it could be what the Chinese call Weituo Kuan (偽托款), a mark that is a copy of an earlier reign mark.
