How ironic. I was at both Home Depot and Lowe's for a project when I asked each what they may have in post caps. Home Depot looked at me with a blank face but the Lowe's girl walk me over to post caps. I had the original with me and my Xoom tablet (better than the I-Pad) with photos of Century Studio 10A camera. I saw the caps you posted but they are too big in footprint and this is all they had.
I just receive an email from the Sign Bracket Store and they sent a photo of the interior of their cast iron cap and dimensions. Just a little too large but close. Below are shots of my original cap, plus the SBS cap (rust colored one). I'm still kicking around the idea of casting one but it's a two part mold based on the original and I just haven't done that in such a long time. Where's Bogart and the African Queen when you really need him....lol.
Thank you gang for the efforts
The original is cast iron with a "O" for ID (2 7/8"+) inside. It's 3 1/16" x 3 1/16" OD by about 7/8" tall.