Bob, Welcome to 'Crosby Fan World'

As to your disc it is best to take the answers in two parts because much of Bing's recorded output circulates and is collected as copies, in both commercially issued 78s, LPs and CDs and as informal copies, whilst there is a separate interest in 'collector's pieces' - original discs of various types such as tests, un-issued pressings and the like. 

if, in 'Wrap Your Troubles In Dreams', Bing sings 'Sang the wrong melody , we'll play it back, See what it sounds like, hey, hey, They cut out eight bars - - - ' then it is one of the best known of the 'fluffs' and copies have been widely circulated in various formats. It was recorded on 9 June 1939.

'Jimmy Valentine' in the normal version, was recorded in the same month, on the 30th June, but it was part of a medley and I personally do not know of a 'fluff' of it. Anyone else any knowledge?

What you have as a disc is another matter. Given the content and the proximity of the dates, I would speculate that it is possibly the work of a Decca studio technician making unauthorised dubbed copies of the fluffs for his own private collection. Others who know more of the nature of these discs and studio practices of the time might, I hope, be able to add more.

In the meantime I suggest you take good care of it.