Another take, pinched from yet another site:
I am a long standing Co-Operative customer and had a long conversation with a higher-up about this issue a while back. Previously, there were the two Visa card types - Visa Debit (previously Visa Delta) and Visa Electron. Visa Debit does not require a merchant check if the account holder has funds available for certain amounts, whereas Electron always requires that the account balance is checked to ensure there are funds available to cover the proposed transaction, which is why some retailers don't accept it (they aren't able to do an immediate check on available funds).
Naturally with most matters banking that annoy and confuse end users, most of the problems stem from contract issues between the merchants and the card issuers (i.e. transaction fees, fraud liability and so forth). The general rule is that Visa Debit is more likely to be accepted than Visa Electron. It is easy to spot an Electron card - they don't carry embossed details like most cards.
Of course, this is much to simple a system to have in place so Visa introduced a third option, a card that requires fully cleared funds like Electron but called Visa Debit, just like the original Visa Debit card. This card can have traditional embossed details, making it indistinguishable from a regular Visa Debit card. Because the merchant contract terms are similar if not the same for this card as Electron (Visa want to replace all Electron cards with this one, but some banks still issue traditional Electron cards), it will act in the same manner as an old Electron card when used to purchase. Except when it doesn't - as it is a Visa Debit card, some retailers will accept it as such even when they do not accept Electron. One of the most notable of these is Tesco when shopping online.
So, to summarise, we now have 3 types of card - original Visa Debit, Visa Electron and Visa Debit "lite" which is supposed to replace Electron and will act like a real Visa Debit card or a Visa Electron card depending on how the retailer interprets it.
In Co-Operative's specific case, as you mention, you have your original PAN (Primary Account Number). This is because Co-Operative (some argue in breach of regulations) have updated a portion of PANs beginning 4508 75 from being Electrons to Visa Debit, although the Visa clearing system will still recognise and process them as Electrons. I would expect that your card is one of these.
TL;DR: Co-Operative decided to make a rebrand a load of Visa Electron cards as Visa Debit cards as per Visa's new marketing push, but the clearing system still recognises the card number as being an Electron. Some retailers will process them as Visa Debit, however.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014