I have had a pressed aluminum plate made for my Manx, I spoke to the people that make them and as Rik has stated, there's no legal size for pre 73 black and white plates. So they've made it using front number plate sizes letters which is perfectly legal (officer )

email (option): horror@blueyonder.co.uk

looking at some common fonts and comparing with the 'official' document

ARIAL & CALIBRI are pretty close, but you need something between Bold and Regular set (I use 'Bold' and it don't look right!)
You would also need to shuffle height and spacing to required 1/4" or so.

Print on paper, cut out and hold in place with PritStick then roll paint on.

I once used AVERY label paper as a mask - DONT - took me ages to get the label off!
You can also get 'low-tack' masking film, but many printer inks wont 'take' (I think a laser printer is ok) - once used this stuff successfully for a 'flame job' as I'm no good with an air-brush.

Better still find a sign writer or one of those guys that do Pub Menu boards ....?

email (option): ginantonik@tiscali.co.uk

Rik says theres no mandatory FONT. The letters must conform to the correct sizes as previously mentioned.

Sorry I misread what Rik said, but the fact is there is no legal size for pre 1973 number plates and letters. Here's the guide lines for reg plates, note it goes into great detail about modern plates but nothing about pre 73 plates except the colour and that the plate can use 3 lines and not just 2.

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/359317/INF104_160914.pdf

If you look at this place that do pre 1973 motorcycle reg plates you'll see you can have 1 3/4" or 2 1/2" letters on the rear plate, both are legal on pre 73 bikes.

http://stores.ebay.co.uk/ceneyservices/Cycle-Pre-1-Jan-1973-/_i.html?_fsub=6955842015

2 1/2" over 3 lines;
http://www.ebay.co.uk/itm/Classic-Vintage-Black-Silver-Pressed-Motorcycle-Shield-Number-Plate-Triumph-/261825619930

1 3/4" over 2 lines;
http://www.ebay.co.uk/itm/Classic-Vintage-Black-Silver-Pressed-Aluminium-Scooter-Number-Plate-/251584511250

email (option): horror@blueyonder.co.uk