Replicas & Miniatures offers two different size acorn nuts cast is resin, the rim bolts are the smaller size. The axle nut was created with the UMM punch above and the lug nuts are hex rod.
The larger R&M acorn nuts are used for lug nuts here:
Splurged and bought this hex punch set and so glad I did. Makes great bolt head/nut detail.
http://umm-usa.com/onlinestore/product_info.php?cPath=21_145&products_id=1870&osCsid=6f323be6fa8fa3c41699f4f32b4c6247
Bolt detail here is a combination of the Tichy bolts and hex punch.