Some original MGA (perhaps most of them) used T-nuts in the floor to secure the seat slide rails with 1/4-inch bolts. My MGA uses wood screws, two or three in each end of each lower rail installed from the top. The Service Parts list calls for HPZ0405 Hex head Pointed tip Zinc plated screws, 1/4-28-UNF x 1/2-inch long, 8 pieces (and spring washers). This implies that the T-nuts are 1/4-28-UNF thread, but they are not listed separately in the SPL. Apparently they were originally pre-installed in replacement floorboards, so not listed or sold separately.

Original T-nuts (the rusty one in photo above) were rather small in diameter, and had three small tacks (to hold them in place when the bolt is removed). Current replacement T-nuts may be larger in diameter with a flat on one side of the flange ("D" shape flange) and still three holes for tacks. The one in picture above came from Clarke Spares And Restorations.

I believe the depression in the wood (above) is not originally machined, but it is caused by machine pressing the flange into the wood. It is common for the flange to have three little spikes to dig into the wood to prevent turning while tightening the bolt. If they have longer tabs they don't need the tacks. McMaster-Carr has them with 1/4-20-UNC threads (not fine threads), long prongs, no tacks, as cheap as $0.09 each in packs of 100.

Addendum August 2, 2022:
Mark Wellard in Australia wrote:
"Attached is a picture of the original t-nuts on my car. They were all attached with tacks that had their points removed. As you can see in the picture, none have any indication of ever having a point. I guess it's possible that they were long enough to protrude into the car and the seat rails flattened them when installed. The t-nuts are UNF, unlike all the replacements I've been able to find".

Looks to me like the small nails had flattened tips (like a screwdriver point) that were bent over like a rivet during installation. Pretty sure these T-nuts were installed in the boards before he boards were installed in the chassis. -- Barney