@Rainflurry 

Thank you so much. I'm glad you managed to decipher my blurb...

In fact, using the 'Stock Shrinkage' COGS option, produces, at times, odd results: Physical Stocks (inventories) are correct but values sometimes produce '0', which is what I would expect by increasing and decreasing the stocks by the same product, albeit via the two different products created, and sometimes produce plus or minus values, which, not being an accountant, I struggle to understand why it is so, though I imagine it may be because the 'average cost' and/or the selling price are different or have changed in time.

More important, for the moment is to understand your suggestion.  

in the "Inventory adjustment account" pull-down menu should I create what you call a clearing account? If so what parameters should I use

for example:

Account Type = 'Current Asset' instead of 'Cost of Sales' ?

Detail Type = 'Stock' instead of 'Supplies and Material COS' ?

I am asking as I am not 100% familiar with terms used in accountancy...

Obviously, in the pull down menu of 'Stock Adjustment Account' we have both Stock (A) and Stock (B). So I could 

1. enter the 'Stock qty Adjustment' form

2. select Stock (A) in the pull down 'Stock adjustment account' menu and decrease the product's inventory qty (for example [XYZ-a] -10)   

3. save and repeat 1.

4. select Stock (B) and increase the product's inventory qty ( [XYZ-b] +10 )

5. save

I don't know whether the above would fall into the "using a clearing account" suggestion or you mean I should create a brand new 'clearing account', in which case I would need to be baby fed with how to do it, in case I should use parameters that are different from those I have listed in the first part of this message.

Hope I am not abusing your patience