Hi Mike,
My immediate thought was it might be one of a few known examples from a 2nd Class Business Sheet which was at least partially missing blue.....and thus blank stamps. However, the two phosphor bands on your item remove that option from the running as 2nd Class would have one band.
I would very strongly expect that your item is from a sheet where the ink ran out. A very extreme version of a dry print of the coloured ink if you will. I seem to recall Mark Bloxham in the past had at least one example that claimed to be a 1p stamp created thus. There happens to be a strip on eBay at the moment that shows exactly this;
https://www.ebay.co.uk/itm/1p-ELLIPTICAL-PERF-MACHIN-UNMOUNTED-MINT-STRIP-WITH-SUPERB-DRY-PRINT-ERROR/333524338583?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649For posterity, the image from that listing is this (I'm sure Richard at Edmonds & Turner won't mind me posting their scan here)
This one looks like it might be an Enschede stamp and since you mention blue gum, its a probability that yours is too....likely from the same sheet. I'd need to see it in the flesh to be narrow it down with any more certainty.
I have similar examples from other values & printers
Hope that helps.
Scott