Abbreviating words is very common in English. (In linguistics, we call it 'clipping'.) For example, bus comes from autobus, fridge comes from refrigerator, and flu comes from influenza.In this part of the world, air-conditioner is also nearly always shortened. However, in Brunei and Singapore, it usually becomes aircon, with no final 'd'.
But in Limbang, the extra 'd' seems to be the norm − I saw it not just on this sign but on two others as well. I'm not sure why people in Limbang (and maybe elsewhere in Malaysia?) use this slightly longer version.
It probably makes no difference to the pronunciation, as I expect the [d] would not actually occur in speech. But some other variant clippings do affect the word quite substantially. When we first moved to Singapore many years ago, my wife made a mistake with the abbreviation of condominium. This is usually shortened to condo, but she included a final [m], which is a bit unfortunate.
Luckily, the presence or absence of a final [d] on aircon(d) does not have such consequences.
