having looked thru the Kew register for October 1851, it does appear that Fred is correct in suggesting that there is just the one Registration that month for Barens Blumberg, which is a p/w - and looks to be a paper backed example, showing a coloured picture of The Crystal Palace. This was allocated Rd. No. 81217 and is 31st October, as mentioned by Fred. Let me know if anyone wants to see this image.
I could be very wrong, but this sardine dish looks untypical of other pressed items from the earlier part of the first period.
During the first lozenge period, the day date - unlike the year and month - is not coded in the form of a letter or number - so you'd imagine quite straight forward to read from the diamond, unless of course worn or poorly stamped etc.
Roy - are you able to post a clear picture of the lozenge?