The Sony A7R IIIA (ILCE-7RM3A) is a quietly released April 2021 refresh of the 2017 A7R III, and a genuinely distinct SKU rather than a rebadge. Sony updated the model mid-life without fanfare, so many UK sellers and even some dealers list it interchangeably with the original - which matters, because the A version is the more desirable body.
Imaging is unchanged from the A7R III: a 42.4-megapixel back-illuminated full-frame sensor, 10fps bursts, 5-axis stabilisation, Pixel Shift multi-shot and dual SD slots (one UHS-II). The A-specific changes are a much sharper 2.36-million-dot rear LCD in place of the original 1.44-million-dot panel, a faster USB 3.2 Gen 2 (10Gbps) USB-C port, and the removal of the Sony logo from beneath the rear screen. Battery life dips fractionally due to the brighter panel; everything else, including menus and autofocus, carries over.
Its significance for the used market is precisely this SKU split: the IIIA commands a premium over the III for its far more usable rear screen and quicker tethering, and it remained on sale well after the A7R IV/V arrived, so late low-shutter-count examples exist. For landscape and studio photographers it remains one of the best value high-resolution full-frame bodies in the Sony FE system.
The essential used check is confirming you are actually getting the A: look for the absence of the Sony logo below the rear LCD, verify ILCE-7RM3A on the base plate label or in the menu system, and judge the rear screen's obvious extra crispness against an original. Beyond that, apply standard A7R checks: shutter count via the EXIF, sensor inspection for oil spots at f/16, port cover condition, and card door latch. Boxes and receipts help here more than usual, since misdescribed original IIIs at IIIA prices are a real UK marketplace hazard.