I know that the fonts can be used to create rubber stamps. I wanted to find out if I create a rubber stamp for each letter and number and create a “stamp kit” from these will I be able to sell my stamp kit? This is a physical form of each letter and I’m not selling the actual font.
Yes you may do that with OFL-licensed fonts. See the OFL-FAQ entry 1.1:
Question: 1.1 Can I use the fonts for a book or other print publication, to create logos or other graphics or even to manufacture objects based on their outlines?
Answer: Yes. You are very welcome to do so. Authors of fonts released under the OFL allow you to use their font software as such for any kind of design work. No additional license or permission is required, unlike with some other licenses. Some examples of these uses are: logos, posters, business cards, stationery, video titling, signage, t-shirts, personalised fabric, 3D-printed/laser-cut shapes, sculptures, rubber stamps, cookie cutters and lead type.
Since you are not distributing the digital font, or any software directly based on it, you do not need to include the OFL license.
Keep in mind, however, that designers naturally deserve credit for their work. So if you manufacture and distribute work based on the work of someone else, it’s proper to acknowledge that. For example, you might include a note with the packages that say something like:
These stamps are based on the font (font name), designed by (designer), and licensed under the SIL Open Font License 1.1 (SIL Open Font License (OFL)). It is freely available from (URL).
A nice benefit of this is that it’s clear - to anyone who wonders - that you’ve done this with the permission of the original designer.