Personally I have no idea and think both these origins are suspect in that the naval college only moved there in 1863 and the prison uniform was first adopted in the 1870's but the last convict transportation was in Jan 1968.
TonyG
I didn't realise we were still taking English prisoners as recently as that
Actually, that was 1868!
Some websites say the "Broad arrow" type uniform was only introduced in the 1870s, but the Perth Town Hall which was built in the 1860s to '70s, largely by convicts, shows evidence of convict influence in the tower windows which are shaped like "broad arrows".
Possibly, the broad arrows were adopted in the then Colonies a bit earlier than in the "Home Country"
In any case, the broad arrow was used as a symbol for government ownership, right up to the end of WW2.
I remember, as a small child eating "luncheon meat" from surplus military rations which were resold by dealers (no "use by" dates, then!).
These had the initials "D" & "D" ( for Department of Defence), separated by a "broadarrow".
We called them "Dad & Dave" after the characters in the old Steele Rudd stories.
There were other names, among the more printable was "Tinned Dog".
It seems, that apart from the government connection, the "broadarrow " was a fairly widely used symbol back in the late 1800s, rather like the "asterisk" & "hash tag" are today.
The town of Broadarrow in WA was named after the symbols a prospector left scratched along the track leading to his "find".
Alas, the deposit petered out, & it is now a "ghost town".
Back on topic, I think the "Dart" means "dirt", especially if spoken in an exaggerated English regional accent.
Remember, back in the 1870s (or even the 1970s), English accents were much less "generic" than today.