The part number and Inspection stamp you posted seem to indicate a Douglas
built product. Douglas inspection stamps used that distinctive D shape with a number inside it tho usually a much longer D. The "hammer" inside is very confusing tho unless it is a stylized 7, which I very much doubt. I would expect it is a process stamp like the HT stamps.
If you look on other parts of the mounting pedestal and find part numbers and inspection stamps it should be possible to identify the manufacturer. If there are more of the Ds but with numbers in them then it is a Douglas
built product. The part number on the large casting should be easy to read but it may have Alcoa stamps. Sheet parts are better for stamps.
Remember lots of companies built parts for other companies so the inspection stamp only shows the manufacturer, not the designer.
View attachment 601715
Typical Douglas stamps look like these samples
View attachment 601713
If you find a stamp that looks like those below, or a variation of them, it indicates the part was inspected after being heat treated. Every manufacturer used similar stamps for this process. An AN stamp means it passed military inspection as well, tho those are much less common.
View attachment 601714
Douglas part numbers follow the same pattern as your number, 199125, but there is no instant recognition of the aircraft model as on Bell, Curtiss, Consolidated and NAA aircraft as the first digit of the part number indicates the drawing size. With Douglas the -1 on the end usually indicates there are both left and right handed versions of the individual part the number is stamped into and this is a right handed (if my rusty memory is correct) version of the part. With some other manufacturers the -1 indicates the first identified/detailed part of assembly 199125. EG Spitfire part numbers all start with the drawing number of the first model the part was used on followed by, in some cases, up to over 2000 dash numbers