Really Right Stuff L-Brackets are excellent; I use them on all the camera I own and test.
This direct from Really Right Stuff:
We just got one of the Sony A1’s in house yesterday from a local customer and can confirm the base plate for the Sony A7RIV (BA7RIV-B) Rev C works, but rev A and B do not.
The L-plate (BA7RIV-L and subsequently BA7RIV L-Set) will mount but the multi-USB port for the Sony remote shutter release is obscured unless the L-plate is slid completely out to the fully extended position*. The Ethernet port and flash-sync also can only be used when the L-plate is in the fully extended position. There is full access to the ports closer to the LCD-side of the camera, whether the L-plate is in the collapsed or extended positions.
All revisions of the Ultra-Light plate (BA7RIV-UL) will fit but also suffer from the same issues accessing the multi-USB, Ethernet, and Flash-sync ports, unless the user fully-extends the plate.
*We didn’t have a Sony-branded shutter release but we were able to plug in a similar connector
We are actively revising the L-plate and Ultralight to remove the secondary QD socket, which will allow for improved port access.
Due to the width of the ports on the camera, we’re unsure at this time how accessible we can make it without compromising the integrity and rigidity of the plate. We will keep you updated as we finalize our design and complete our testing.
DIGLLOYD: good news for those changing over. The Sony A1 plate looks to be in progress, but awkward to design. I myself can live with the restrictions for a time.
Update: I’ve been using the ultralight Sony A7R IV L-bracket on the Sony A1 just fine. It’s not ideal for the ports on the left side, but I never use those.