I think that the variations in the labels are because different a printing press was used.
Looking at the catalogue letters and numbers, the variation without the 45 in a triangle use a small font. Compare the cat. number on the scan uploaded by clearbluesky with that uploaded by hantsguys.
This print variation (using a small font) is common across nearly all the DECCA group labels and occurs on some releases from early 1964 until at least 1966. Examples of that font used for cat. numbers can be found on ATLANTIC, BRUNSWICK, CORAL, DECCA, LONDON and RCA records released at around that time.
Thanks for that piece of info Okun69. Had a quick look through the 45cat UK RCA discography and it would appear that our earliest release minus the 45 in an upside down silver tri-angle/arrowhead is 'Such A Night' by Elvis on RCA 1411 from August '64 and the latest I've found in Elvis again with 'Blue Christmas' on RCA 1430 from November '64 with a few similar ones noted between these dates. There is a mixture of both label designs from this period so maybe RCA had a certain number printed up but decided not to change the design permanently once stocks had been used up.
Label variation added. I note that on the copy I've added doesn't have 45 in an inverted silver tri-angle anywhere on the label. I did think it might be a later issue but looking through the UK RCA discography most releases appear to have the silver 45 somewhere on the label right up to when the label changed from black to orange. So I assume it's just a variation on the usual RCA label design of the time.