- SMLE No. 4 Mk. 1, 1/2, Mk. 1/T, Mk. 2, and perhaps other variations use the same bolt head pattern. Mk. 1* uses a different pattern bolt head. While the Mk. 1 bolt had cannot be used on Mk. 1*s, can Mk. 1* bolt heads be used on Mk. 1s?

- Can No. 4s accept No. 3 magazines? If they cannot, please explain why not.

- While No. 4 magazines are detachable, the Mk. 2s from Long Branch (Canada) made in December 1954 I owned had sear/trigger springs that were so strong that magazine removal nearly required both hands and a tool. In World War II were No. 4 magazines routinely swapped out during firefights, or were magazines filled by way of 303 stripper clips - as were M1903s and M98Ks?

- What was the British army's expectation of the length of service life for No. 4 Magazine boxes? . . . for magazine follower springs?

I will answer what I can, I don't believe there is any interchangeability with the #1 and #4 bolt heads, there are obvious differences. #4's can accept #1 mags if the full-length guide rib on the back of the #1 mag is milled off up to the latch cut. The Brits did this when necessary and I did one in my mill just last week, it works fine. You could grind or sand it off also. The Brits did not change out mags, they believed in recharging thru stripper clips as needed. If you notice on the #4 bottom metal, there is a loop near the front screw that sometimes used a tiny chain to secure the mag to the rifle. All rifle magazines can be considered "consumables", but since the Enfield's mags were removed rarely, I imagine that their lifespan was very long, many of the early ones were serial numbered to the rifle, if I remember correctly. I think the life of the mag springs was quite long also, but like anything, Armorours certainly had them on hand. Overall, the Enfield was/is a very robust and reliable battle rifle, designed by American James Paris Lee. It took me many years to come to appreciate the design, but now I have several and enjoy them very much.

Yes, the #4 MK1* bolt heads can be used on the MK1 and all other #4 variant rifles and vice versa; ;the only difference is the way they are released. Make sure you understand your rifle nomenclatures; there is no #3 Lee Enfield; that is actually the Pattern 14.
Of course, the #1s and #4s bolt heads are nothing like each other.
Lee Enfield mags were not replaced in combat like modern magazine rifles. The rifles were filled with stripper clips, which can only be done whilst the mag is affixed in the rifle. They were expected to last the life of the rifle, although spares were stocked.

Lee440, thanks kindly for the info. I'm in need of a #4 mag and have an extra No1. Never bothered to see why it didn't fit and noted the aftermarket folks made a model for each. Sounds like a little time at the mill this weekend.