There are numerous small differences above the colour. The original card is rougher in texture and slightly darker in tone. The overall impression of the reproduction is slightly blurred and individual marks are noted in the four areas of the card; VR, Indicia, Heraldry and Text.
The scans below were made at the same time using a black and white reference card for tonality. Even so, there is a slight colour shift from the actual cards, mostly affecting the 'bluish' feel of the original.
There is a small notch in the left side of the crown's lower band 'edge' in the original. The inner lines of the thistle flower head are much more defined in the original as are the pearls arcing across the top.
The reproduction has a small stroke above the "G" of "POSTAGE" joining the frame lines. The shading lines of the face are coarse and broken in the reproduction.
The top of the crest is well defined in the original, but solid in the reproduction. The fifth line in the lower vertical arm of the cross is shortened - other shading lines are different as well. The facial shading, eyes, noses and tongues, are less detailed in the reproduction.
The full stop of "1890" has a leftwards spur in the reproduction. The delicate top curl of the inner line of the "L" in "London" is a solid 'arrowhead' in the reproduction. All letters appear heavier with the dot over the "i" of "Jubilee" being notable larger than the original.
According to the book "Collect British Postal Stationery" by Alan Huggins and Colin Baker these cards were definitely reproduced. From page 60 under the card entry below: "Reproductions of this card were produced in 1966 and 1990, the latter for the Stamp World London 1990 Exhibition"
The card's entry in the book is: "CP26 1890 1d. carmine stamp L4. Guildhall Postal Jubilee card format CF8, thin buff card size f".
Mr J. Silkin tells me that the Great Britain Philatelic Society is organising an exhibition of the stamps of Great Britain, to be held in the Crypt of the Guildhall from 16th to 23rd July next. This exhibition will be part of the City of London Arts Festival, which is held annually. It will cover all periods of British philately from pre-adhesive material up to current varieties and should provide visitors with an opportunity to see many unusual and exceptional items. There will be no trade stands but it is hoped to provide covers and postcards cancelled with the canceller used at the Guildhall in 1890 for the Penny Postage Jubilee.
"I've been through the catalogue, and the only mention is the use of the 1890 cancellor [of the type P-4] on all mail posted in the Crypt during the exhibition. No mention of any stationery."
"The reproductions were not connected with the GBPS exhibition at the Guildhall Crypt, but produced by someone in the trade and emerged after the event. At the time, some regarded them as fakes, but were actually facsimilies, not intended to deceive. One or two exist in blue, instead of the normal red. He added that there are constant varieties on these cards, indicative of their common origin from a single 'master'."
So ... who was the source of these 'reproductions'?
In 1985 the Philatelic History Society [1984-1992] induced its members to renew their subscriptions for the second year using a self addressed, pre-canceled imitation of the Guildhall Jubilee post card. Addressed to Victor Short, editor of their "Philatelic Paraphernalia", it included a space for listing collecting interests. Although that journal reached No. 29 in 1992, there seem to be no holdings in major libraries.
The card seems to have been reproduced from an actual Guildhall Jubilee Post Card having the H-1 type [directly handed back] of cancel, single color printed on an off white card stock. As the 'stamp' represented had long since been invalidated, a current Queen Elizabeth II value would have been needed if any were returned as cards. An interesting idea, about 5 years early for the centenary of the card.