Thorens TP-16 Tonearm cartridge mounting issue

Recently, I have acquired a Thorens TD-160 MKI equipped with the original TP-16 tonearm and headshell. I am using Dynavector 10x5 and Grado Gold cartridges but would like to try a Reference Grado in the near future. The problem lies within the headshell of the TP-16 tonearm, as it only allows for mounting from underneath the headshell and there are no holes on the top. After close examination, it seems I can remove the lid of the headshell but there remains a top that lies just above the threaded cartridge plate. Ideally, I would like to have 2 slots located above the headshell plate for cartridge installation and adjustment, this would make installation for Grado Reference and other cartridges much easier.

Does anyone know if there are other headshells that I could fit directly on the end of a TP16 with open slots on the top? Maybe a MKII headshell would work but I am really not sure?

Here is a link to a picture of a TP16 tonearm-
http://www.vinylengine.com/library/thorens/tp16.shtml

Thanks to everyone for your input.

Cheers,
Chris