You have a couple of options when trying to assign values with a constant and variable. It sounds like the variable will be manually entered by an end user.  In cases like this, you can use Workflow to assign the constant and then have the end user enter the variable at a later time.

A stamp may not be the best idea here as stamps don’t use tokens the same way that metadata does.  If it would fit your process to use fields to store that value, it may be a little cleaner to apply metadata rather than stamps and this process may even become worth automating using Workflow.

If you are looking to automate this process and the variable will follow a relatively simple pattern such as what was explained below (1234, 1235, 1236), you can use workflow to enter both the constant and the variable.  This Presales Place article shows how workflow can be used to automatically apply a number and name to folders.  This would have to be slightly modified, and will be even easier to set up, as one of the components is a constant in this case, and both of the components are variables in the example used for the article.