Converting integer to binary for SPI transfer - raspberry-pi

Question

I am trying to convert an integer to 3 bytes to send via the SPI protocol to control a MAX6921 serial chip. I am using spi.xfer2() but cannot get it to work as expected.
In my code, I can call spi.xfer2([0b00000000, 0b11101100, 0b10000000]) it displays the letter "H" on my display, but when I try and convert the int value for this 79616, it doesn't give the correct output:
val = 79616
spi.xfer2(val.to_bytes(3, 'little'))
My full code so far is on GitHub, and for comparison, this is my working code for Arduino.
More details
I have an IV-18 VFD tube driver module, which came with some example code for an ESP32 board. The driver module has a 20 output MAX6921 serial chip to drive the vacuum tube.
To sent "H" to the second grid position (as the first grid only displays a dot or a dash) the bits are sent to MAX6921 in order OUT19 --> OUT0, so using the LSB in my table below. The letter "H" has an int value 79616
I can successfully send this, manually, via SPI using:
spi.xfer2([0b00000000, 0b11101100, 0b10000000])
The problem I have is trying to convert other letters in a string to the correct bits. I can retrieve the integer value for any character (0-9, A-Z) in a string, but can't then work out how to convert it to the right format for spi.xfer() / spi.xfer2()
My Code
def display_write(val):
spi.xfer2(val.to_bytes(3, 'little'))
# Loops over the grid positions
def update_display():
global grid_index
val = grids[grid_index] | char_to_code(message[grid_index:grid_index+1])
display_write(val)
grid_index += 1
if (grid_index >= 9):
grid_index = 0
The full source for my code so far is on GitHub
Map for MAX6921 Data out pins to the IV-18 tube pins:
BIT
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
IV-18
G9
G8
G7
G6
G5
G4
G3
G2
G1
A
B
C
D
E
F
G
DP
MAX6921
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
IV-18 Pinout diagram