Hello Rosa, there are two versions of the Double Outline that have this style 9 (and 6). The original Double Outline font was created by the Spacerite company at a metal alphabet and this is likely the Spacerite version. Unfortunately, there is no digital version of the font currently available. The characters changed quite a bit at different sizes, so it is a difficult font to make, as many versions will need to be made in order to match the font at all sizes. However, there will be an MLC version completed hopefully in the next 3 to 6 months.
There was also a plastic stencil press alphabet released at some point to mimic the Spacerite Double Outline, but it is exceedingly rare and which company produced it is unknown. Other plastic Double Outline alphabets had 6s and 9s that connected back in to the themeselves instead of leaving a gap.