[zastava128]

Well I thought about the monitors, as well as small Adriatic patrol boats, but I think we're already going to give Don and Andy an awful lot to do...

Anyway, back to the artillery:

The 76.5mm guns were indeed labeled "80mm".

I've checked about the Royal Army. It used the following guns:

Divisional level

65mm French M.1906 mountain gun - unit 21 in the OOB
75mm French M.1897 field gun
75mm French M.1912 field gun
75mm Serb M.1907 field gun
75mm Austro-Hungarian Škoda M.15 mountain gun
75mm Czech Škoda M.28 mountain gun - unit 54 in the OOB
80mm Austro-Hungarian M.5/8 field gun
80mm Czech Škoda M.28 mountain gun - unit 133 in the OOB
100mm Austro-Hungarian M.14/M.16 howitzer (two very similar models) - unit 147 in the OOB
100mm Czech Škoda M.28 howitzer - unit 135 in the OOB
105mm French M.16 howitzer

Army level - there were no corps

105mm French Schneider M.13 long gun - unit 148 in the OOB
105mm M.15/26 (modified 104mm French M.15, range increased to 15.6km)
120mm French Schneider M.10/15 howitzer
150mm Austro-Hungarian Škoda M.14 howitzer
150mm Czech Škoda M.28 long gun
155mm French Schneider M.10/15 - unit 150 in the OOB

Anything else (various 150mm long guns, 150mm howitzers, 220mm guns, 305mm guns) was attached exclusively to the Supreme HQ.

Also, they did acquire 37mm Škoda AT guns in 1939.

Obviously there's no need to cram everything of the above in the OOB...