I would be interested in any information that anyone has on Indian bolt heads for the 2A / 2A1 series.

To the best of my knowledge NOBODY made incremental bolt heads for SMLEs anywhere until Lithgowicon made a set in the early 1950s, just before the L1A1 came into service.
Viz:

Part Number Body length Size No.
B1/BAA 3425 0.635” 5
B1/BAA 3426 0.636” 6
B1/BAA 3427 0.637” 7
B1/BAA 3428 0.638” 8
B1/BAA 3429 0.639” 9
B1/BAA 3430 0.640” 0

The entire "engineering premise" of the SMLE involved incredibly stringent gauging at every stage of manufacture. Furthermore, and I believe this is critical, the method of arriving at he final chamber form included firing of "special" high pressure cartridges and a certain amount of careful reaming after the barrel was fitted to the receiver and bolt.

From SA 462

Chambered. first. - The chamber will be gauged with gauges smaller in diameter than the finished size, and the extractor way gauged for depth and position from flat; also the position of the key for slot for block, band, foresight, will be gauged.The bore will be examined and the barrel spun.

Sights, fitted. - The barrel will be submitted with the sight bed assembled, and the block, band, foresight, fitted. It will be examined for fit, and gauged for the position of block, band, foresight; bed, back sight; and sight axis pin hole; and size and depth of fixing screw hole, also for height of block, band, foresight, and figure of dovetail. The exterior will be gauged at the muzzle, radius, and breech end, also for the figure of the reinforce.

Proof. - The barrel will be submitted with the action attached for proof. The distance from end of bolt to face of barrel will be tested with a cartridge head gauge .067-inch. The proof will then be carried out with a proof cartridge, the charge being about 33 grains of No. 9 cordite, having a Service bullet of 215 grains, giving a mean pressure not below 24 tons per square inch, after which the action will be examined; and the barrel, body, bolt, and bolt head will be marked with the proof mark.

Browned and finished chambered. - The barrel will be examined for browning and bore (which should be burnished with wire gauze), and to see that the entrance to chamber is rounded, chiefly at bottom, The bore will be tested far straightness by being spun in the testing machine on a rod with two bearing points, one at the breech end and the other at the centre of the bore; the muzzle end will be allowed a total lateral movement of .006-inch. The chamber and bore will again be gauged. The bore must be parallel from end to end, and take a .303-inch or .304-inch plug, and reject a .305-inch plug; the receiving: plug, according to the diameter of the bore to run, The barrel will also be gauged for length.

Barrel with body (breeched up). - The barrel will be submitted for view assembled to the body, the latter being examined for browning and for polishing of the bullet lead. The barrel must be well bedded down into the body. The barrel will then be gripped in a breeching-up vice, and the position of the flat on the barrel gauged from the body. The breeching-up will be tested by means of a hand-operated spring wrench, which, when pulled to completely depress the spring, should not unscrew the body. A pull of about 30-lbs. applied 12-inches from the axis of the movable arm of the wrench being required to move it, and about 55-lbs. to completely compress the spring; the axis of the movable arm being 2,32-inches from the axis of the body. The position of the flat on the barrel will again be gauged, and the barrel and body spun.

What I gather from this is that the firing of the “special” proof cartridge was not just to see if it all held together, but to “pressure forge” the chamber to almost final dimensions: interesting technique indeed.

I don’t have the equivalent document for the No4, but I would hazard a guess that there were a few procedural differences.