First off - 73 is incorrect for The Hastings and Prince Edward Regiment: - 73 was the 'Unit Serial Number' for 1st Medium Battery, R.C.A., C.A.S.F. - from 1 Sep 39 to 1 Jun 40 - after that Serial 73 was not assigned to another unit - 33 was the 'Unit Serial Number' for The Hastings and Prince Edward Regiment (CASF & CA(A)) - from 1 Sep 39 to 15 Oct 45
Secondly - 754 is correct for the HLI of C - 754 was the 'Unit Serial Number' for The Highland Light Infantry of Canada (CASF & CA(A)) - from 24 May 40 to 15 Jan 46
The use of the 'Unit Serial Number' with the addition of a backslash followed by the number 1, came in to use sometime after the standing up of First Canadian Army (approx. 1 Apr 42), but when exactly this occurred, I am unsure of at this time. The reason for the addition of the backslash followed by the number 1, was of an administrative nature to differentiate between units of the Canadian Army Overseas and those of the British Army. This did not affect units of the Canadian Army (Active) serving within Canada, but upon transfer to the United Kingdom for service with the Canadian Army Overseas, the backslash followed by the number 1 was added to their ‘Unit Serial Number’. This also, did not affect those units that were raised overseas under Provisional War Establishments to cover experimental and temporary organizations and special courses of instruction under the authority of either Canadian Military Headquarters (London) or the Army Commander, these units continued to be identified by a number that was prefixed by the letters ‘CM’ (Canadian Military), ie: CM-xxx, which was assigned to them upon authority of their raising under either CMHQ or the GOC-in-C First Canadian Army. Some of these temporary units/organizations were eventually ‘Called Out’ and ‘Placed on Active Service’ under an Order-in-Council, at which point they were authorized under a ‘Unit Serial Number’ and in most cases, a new designation.
This means of identifying units of the Canadian Army Overseas, was purely administrative and after its inception, was used in such things as Canadian Military Headquarters Administrative Orders and in Canadian Army Overseas Routine Orders. It also appears in use in Part II Orders issued by Canadian Section, General Headquarters, 2nd Echelon, in the United Kingdom, Sicily/Italy and North West Europe.
Starting with the Canadian Army involvement with Operation
HUSKY, the 'Unit Serial Number' with the addition of a backslash followed by the number 1, was also used as a form of ‘Embarkation’ number, normally either chalked of painted in white on the front of a unit’s vehicles and were normally only carried on the vehicle for a brief period before embarkation, on the voyage and for a brief period after landing. I have found one clear example of this for the landing in Sicily, which I have included as Example 3 below.
By the time of Operation
OVERLORD, this use of the 'Unit Serial Number' with the addition of a backslash followed by the number 1, had involved a bit more, as can be seen in Example 2 below, where besides the 'Unit Serial Number' with the addition of a backslash followed by the number 1 being used as an ‘Embarkation’ number, the number and type of landing craft in which the vehicle was to travel, was added along side the 'Unit Serial Number' with the addition of a backslash followed by the number 1, and in this case reads
733/1 1712 LCT (IV).
Although partially obscured, in Example 1, this same system of ‘Embarkation’ number, using the 'Unit Serial Number' with the addition of a backslash followed by the number 1, followed by the number and type of landing craft in which the vehicle was to travel, is used.
Example 4 below, is an example of the use of a 'Unit Serial Number' with the addition of a backslash followed by the number 1, as an ‘Embarkation’ number.
As Ed pointed out, the 'Unit Serial Number' with the addition of a backslash followed by the number 1, also came to be used as a means to identify unit ‘Baggage’.
Example 1:
(see attached)
Credit: Lieut. Donald I. Grant / Canada Dept. of National Defence / Library and Archives Canada / PA-132886
Caption: Gunners with a Priest M-7 105mm. self-propelled howitzer of 34 Battery, 14th Field Regiment, Royal Canadian Artillery (R.C.A.), France, 20 June 1944.
The 'Unit Serial Number' for the 14th Field Regiment, R.C.A. was 707
Example 2:
(see attached)
Credit: Lieut. H. Gordon Aikman / Canada Dept. of National Defence / Library and Archives Canada / PA-140192
Caption: H/Major J.W. Forth, chaplain of the Cameron Highlanders of Ottawa, assisting the Regimental Aid Party of the Cameron Highlanders of Ottawa in loading a wounded soldier onto a jeep near Caen, France, 15 July 1944.
The 'Unit Serial Number' for The Cameron Highlanders of Ottawa (M.G.) was 733
Example 3:
(see attached)
Credit: Lieut. Jack H. Smith / Canada Dept. of National Defence / Library and Archives Canada / PA-114511
Caption: Infantrymen of The Hastings and Prince Edward Regiment in a Universal Carrier advancing on Nissoria, Italy, July 1943.
The 'Unit Serial Number' for The Hastings and Prince Edward Regiment was 33
Example 4:
(see attached)
Credit: Lieut. Ken Bell / Canada Dept. of National Defence / Library and Archives Canada / PA-183737
Caption: Personnel of the 17th Duke of York's Royal Canadian Hussars in their Humber Mk. IV armoured car in Normandy, France, 18-20 July 1944.
The 'Unit Serial Number' for the 7th Reconnaissance Regiment (17th Duke of York's Royal Canadian Hussars), C.A.C. was 1102
Hope this helps ...
Cheers