Given the age of the antenna, it's possible that it's engineered for the older UHF band (CH-14 to CH-83). Still, it looks like it's in great condition. I don't know the make or model number, but a 14 director Yagi should give you the gain you've estimated.
Here is the output from "VK5DJ's YAGI CALCULATOR"
Code:
VK5DJ's YAGI CALCULATOR
Yagi design frequency =600.00 MHz
Wavelength =500 mm
Parasitic elements contacting a square section metal boom 25.00 mm across.
Folded dipole mounted same as directors and reflector
Director/reflector diam =6 mm
Radiator diam =6 mm
ELEMENT LENGTHS AND SPACING
The abbreviation "IT" means "Insert To", it is the construction distance from the element tip to the edge of the boom for through boom mounting
Reflector
262 mm long at boom position = 30 mm (IT = 118.5 mm)
Radiator
Single dipole 232 mm tip to tip at boom posn =130 mm (IT = 103.5 mm)
Folded dipole 257 mm tip to tip at boom posn =130 mm (IT = 116.0 mm)
Dir Length Spaced Boom position IT Gain Gain
(no.) (mm) (mm) (mm) (mm) (dBd) (dBi)
1 230 37 167 102.5 5.4 7.6
2 227 90 257 101.0 6.9 9.1
3 225 107 365 100.0 8.1 10.2
4 222 125 490 98.5 9.1 11.2
5 220 140 630 97.5 9.9 12.1
6 218 150 779 96.5 10.7 12.8
7 216 157 937 95.5 11.3 13.4
8 214 165 1102 94.5 11.8 14.0
9 213 172 1274 94.0 12.3 14.5
10 211 180 1454 93.0 12.8 14.9
11 210 187 1641 92.5 13.2 15.3
12 209 192 1834 92.0 13.6 15.7
13 207 195 2029 91.0 13.9 16.1
14 206 197 2226 90.5 14.2 16.4
Spacings measured centre to centre from previous element
Tolerance for element lengths is +/- 1 mm
Boom position is the mounting point for each element as measured
from the rear of the boom and includes the 30 mm overhang.The total boom length is 2256 mm including two overhangs
The beam's estimated 3dB beamwidth is 31 deg
A half wave 4:1 balun uses 0.75 velocity factor RG-6 (foam PE) and is 187 mm long plus leads
Here are some construction details for a folded dipole
Measurements are taken from the inside of bends
Folded dipole length measured tip to tip = 257mm
Total rod length =543mm
Centre of rod=272mm
Distance HI=GF=106mm
Distance HA=GE=133mm
Distance HB=GD=161mm
Distance HC=GC=272mm
Gap at HG=10mm
Bend diameter BI=DF=35mm
If the dipole is considered as a flat plane (see ARRL Antenna Handbook) then its resonant frequency is less than the flat plane algorithm's range of 10:1