SATA has a slot between power and signal connectors. SAS doesn't have that slot. SATA-only receptacle has a protrusion that goes into that slot. As SAS doesn't have that slot it is not possible to plug a SAS drive into SATA-only receptacle.
The opposite is possible -- SATA drive can be plugged into SAS receptacle so those are SAS/SATA physical connectors.
The actual controller that those drives are going to be connected to are separate question. Many SAS controllers will also work with SATA drives. The opposite is not true. AFAIK there are no reliably working USB<->SAS docking stations. There are _SOME_ that claim to be SAS but those are hit and miss -- it is impossible to tell if they'll work with a particular drive. SATA<->USB ones are aplenty, cheaper than dirt, and almost all of them work.
Another issue is that SAS drives don't always have power of two sized sectors. Although many are usual 512-bytes per sector there are still many with 520/528/536 etc. bytes sectors. Many of them (but not all) can be re-formatted to 512-byte sectors but it is not for the faint of heart :)
SAS is SCSI with serial physical interface, akin to PCIe that is PCI with serial physical interface. SATA is, eh, ATA with serial physical interface so it is totally different device.
I can't seem to note any physical (e.g., dimensioning, connector placement,
> fastening) differences between the two types of drives (other than the
> obvious difference in connector geometries).
>
> I'm trying to understand why there would be different *sleds* for the
> two types (SAS vs SAS/SATA) as, mounted, there seems to be no difference. >
> All I can imagine is some mechanism that prevents one type of sledded
> drive from being installed in a machine that doesn't accept that type
> of drive (I could see a SAS drive buggering a SATA-only mating connector). >
> And, of course, a machine that can't take advantage of the SAS interface
> would be foolish to accept such drives.