I was working from the centre-line of the scrum outwards

I think "quarter-back" would be quite sell-able as a position to the Americans for a hooker as well instead of "dummy half".

5/8 came from NZ (as far as I am aware)fly-half from Wales (came about at a time when rugby was essentially a bunch of continuous fluid scrums). The position on the field were originally slightly different 5/8 stood deeper than fly half and there is also a notion of a second 5/8th which tends to be a secondary playmaker as much as a centre. Roles of the Positions in the NH and SH varied cobsiderably in the early years hence the difference in names.

They call it the three quarters in union too which is strange.

12 Centre
13. Centre
11 Winger
14 Winger
15 Fullback

If you add 5 and 8 you get 13.. just sayin like..

Fly half/stand off also called out half or outside half in some places down under.

Out 5/8 is sometimes used in Union to describe Inside Centre (12).

Not sure where the description of three-quarters came from as that has been around for donkey's years in both Union and League.

Why is it strange?

Remember in both codes the numbering used to be the same (same us now in league). Union swapped their numbers round at some point in history. Can't remember why now. Other teams used letters to try and confuse the opposition (Welsh sides originally, but more recently leicester but they have since reverted to numbers).

I know a Standoff called Dave

There not halfs, 5/8ths, flyhalfs or scrum halves theyr'e generally refered to as orrible little SH!T$