The only certain solution that I can see for this at the moment requires (at least) two Monarch processes.
In the first, create a summary using only the Property as a key field. Export the summary, and then use that output as the input for the second Monarch pass.
In that model, build a new Numeric calculated field using the Rowno() function, and you'll have the numbering that you want.
But then I imagine that you may need to export those values and use that output as an external lookup source for some other purpose.
If you get a chance, (plug alert) watch the Invisible Data webinar. Using Rowno() and Recno(), possibly with two passes, will solve this problem for you and your users.
Firstly, define a sort order on the table for the key values in the right order, then a null filter with advanced properties for unique key values. Then define a calculated field = Rowno(). This will give you the 1, 2, 3, you want.
Export the table filtered and sorted as such, and then use this export as a lookup joined on the key value. This will bring in 1 against every instance of A, 2 against every instance of C, 3 against F and so on, changing on a month by month basis.
This can all be automated neatly in two projects and a batch file.