Thanks for the information ! So they're not ancient SLCs, more like 10 years old.
Let's see if maybe your RSLogix 500 and RSLinx are older than the controller firmware. Post what version numbers you have for both of those.
I remember an issue in 2006 or so where if you used RSLogix 500 version 7.10 or newer to download to an SLC-5/05 with Version 10 firmware, then older versions of RSLogix 500 could no longer connect to the controller. The error code was something like "corrupted processor".
RA Knowledgebase article 36596 describes that issue.
The solution was that you needed RSLogix 500 version 7.10 or newer, or you could define the CPU as one with the pre-v10 firmware and then re-download, in order to allow your RSLogix 500 version 7.x or older colleagues to regain access.
Your info about seeing version 3 as the firmware version makes me scratch my head. Let's look at it again.
I have an SLC-5/05 controller handy. In the RSLinx browse, it shows up with the descriptor
SLC-5/05, 1747-L551/C C/11 - DC 3.46
That means it's Series C hardware, which can support firmware 10 and up and has the 10/100 Ethernet plug. "C/11" means the operating system is OS501 Series C, Firmware Revision 11. The Ethernet daughtercard is version 3.46
But if you right-click and select Device Properties, you see some misleading CIP Identity Object information:
Vendor: 1 (Allen-Bradley)
Product Type: 12 (Programmable Controller)
Product Code: 176 (SLC-5/05 model 551)
Product Revision: 3.011 (WTF ?)
My best guess is that the CIP Identity Object is "3.011" because this is a Series C Firmware 11 controller.
I've said it before: SLC-500 firmware revision naming and numbering is partially due to the late Jon Spry, who introduced the Milwaukee product team to the charms of aged tequila in the early 2000's. We miss you, big guy.