Since the #4mk1 rifle was the primary battle rifle of the Commonwealth, nearly all have been through an "armorer's" shop. Acceptable stocks were of walnut, birch, and beech, There may not be a consistancy, but the rifles stock sets were interchangable. Wood variations are normal. Keep in mind that Savage and Long Branch in Canada traded parts often, so it is common to see cross parts on either rifle.
A miss matched bolt may not be an issue to a shooter. A more desirable collectable piece would be all matching, but that is not a requirment. If the rifle went through an FTR (Factory Thorough Repair) at either Fazakerly or Maltby factories, then the possibility of a miss numbered bolt would be high. The rifles that went through FTR were brought uo to spec, and were nearly completely new. Damaged wood, swivels, front sight guards, rear sights, or other parts would be replaced with "serviceable" parts, either new or used.
Concerning headspace, the #4 rifles had 4 different bolt heads, numbered 0, 1, 2, and 3. 0 was the shortest, and 3 was the longest. The rifle headspaces on the case rim, and not the shoulder, like other rifles. So to tighten a headspace, the armorer would change the bolt head only. If a bolt body were replaced, then the origional was damaged, lost, or something else bad. I have FTR'ed #4's that are all matching, so the armorers did not haphazardly sling parts around just for the fun of it. I had one rifle that was FTR'ed at both Fazakerly and Maltby that was all matching. I sold it to a friend because I had 2 other Fazakerly #4mk1's already.
The Savage is a good deal, but see if they will let you put one in lawaway, and wait for the price to come down. I would rather pay $179 than $229 any day.
Here are 4 of my rifles.