Overmantel frames in Gibbs's 1728 book as a possible source of Salisbury doorways
The fairly authoritative Book of Architecture by James Gibbs, published in 1728, contains, for instance, this illustration of overmantel frames for pictures, which might in principle explain the appearance of such doorways in Salisbury. If, say, some enterprising builder decided to economise and used these elements, taken from a wealthy house where they might have been replaced with more modern ones