The spacer (#28) was simply used to fill in the different width of the stamped Vs milled backsight pivot dimensions. It had absolutely nothing to do with "sprung, or spread" receivers.
Looking at the pictures of your (very tasty) #5 it seems to have the "L" flip sight. You may want to concider replacing it with a ladder type with ranges from 25 (unmarked) & (marked100)~800yds as it'll be far more precise for the brush use you were mentioning in the earlier thread.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”