Hi, 44 AMP,
Yes, there was some .455 ammo, but even then it was scarce. I was around (then) Interarmco quite a bit at the time (my now wife worked there) and both .455 and .380 (the British revolver ammo, not .380 ACP) were pretty scarce for the reason I mentioned - they just didn't make a lot. I got some .455 and .380 but it was not like .45 ACP and 9mm P with literally tons available. The limited .455 supply dried up very quickly, but Interarmco and others had already been having those guns modified. The conversions were mostly done in the US because had they been done in the UK, the guns would have had to pass proof at .45 ACP levels and (IMHO) some would have failed. So they were proved for .455 when they were sold out of UK government stores, then converted after they reached the US.
I have a Mk I and a Mk VI both of which are still .455. It is available and I can make ammo from .45 Colt or .45 AR if necessary.
The boxes, BTW, hold 12 rounds, so $1.70 a box would be pretty expensive for the time.