Bolt head number sizing was done as a manufacturing shortcut. Whatever size was needed was used. (0 thru 3)
Replacement was just a side benefit, although when a rifle goes out of headspace it's due to wear elsewhere.
If there's nothing wrong with the rifles now, don't worry about it. I don't think very many of us could wear out one of these in our lifetimes.
Thanks for the responses, guys. My assumption was that if the bolt head lug over turned the bolt body (as shown in the Canadian tech manual), then head space would be too long, necessitating going to the next size head.
In other words, longer bolt heads had a longer thread.
So, what you're saying is that for the most part, the threaded portions are all the same, it's the body of the head that varies in length...
Out of over a dozen new unissued No. 4s I only have one with a 2 bolthead. The rest are 0 or 1. I did not pick them with any thought of the bolt head number, most came straight from the importer/distributor sight unseen. These include all the British makers and Long Branch. The 2 head is on a 1943 Faz that, although new, looks more like a dispersal factory SMLE in quality and woodwork.