GB MINT EII Machin 9p 2B blocks varnished & unvarnished - snow storm effect

I pulled this from a Machin collection.  I do not have the lamps to check the fact - but the album notation said the block without the selvedge had a 'snow storm' effect due to lack of varnish and indeed it does.  I see in the specialised book varnished is listed as U174 and unvarnished as U175.  That is the extent of my knowledge in the item.