This was on the firewall, 1924 Touring

What is this bolt for?

The top one are bits of a broken chain pull lamp switch...

What is the thread size of the bolt?

The top picture is the part that was on the firewall, I don't know about a lamp chain pull but that is what it looks like

That's a "Holy Bolt" Batman! (couldn't resist two puns in one). Likely flywheel bolt, might not be a
T though.

Doug, that bolt is not like any bolt I have found on a T. The black thing next to it looks like a cushioning device to prevent hammer blows marking T parts.

Allan f from down under.

The lower photo appears to show a nearly period-correct human male left hand. Patina and injury are very authentic, but tape is modern vinyl type. So I rate this component as a "bitsa".

Oh, the bolt ? Beats me.

Looks like right hand thread to me.
Electric tape looks like a band-aid.

I had to chuckle when I saw the electrical tape on the thumb! I have used electrical tape as bandages for most of my life. I used to always carry some in my pocket for work, almost always still have some handy. I figure the inside of that tape roll is much more sanitary than the world around any cut I get. Keeps dirt out, blood in. What more do I need?
One word (sentence or two?) of caution for anybody that may want to try this. Electrical tape by its very nature is stretched as it is rolled onto itself in the factory, and stretches a bit when it is pulled off the roll, so it WILL shrink after you wrap it around a finger, arm, or anything softer than a lead pipe. Be very careful not to get it too tight as it can cut off blood flow (like a tourniquet). Such a thing could cause loss of a finger, or worse. Just be careful and keep an eye on it. Loosen it if it becomes too tight.

On the humorous side. I often had tape on a finger or two, because a lot of the work I did was risky, or had very sharp edges. I was generally very careful, and therefore never lost a finger. However, I often had cuts. Sometimes, people would notice the electrical tape, and make a comment about it. Because a lot of the work I did was also with electronics and power systems, I would often tell people that because I worked with powerful electrical systems, my blood was electrically charged and I therefore had to use electrical tape on cuts.
I got a lot of strange looks from people that way. Usually they were followed by a laugh as they realized I was joking.

I dont keep bandaids in the garage so rather then get blood all over my wire harnesses I just grab the electric tape. Im sure some can relate...after wiring the headlight connector I was tightening the thimble onto the connector and the connector spun taking a chunk out of my thumb, those damn tits are sharp! By the way I just finished installing all the harnesses I bought from Langs and they are very nice and fit spectacularly. Doug