Hi
Have you considered contacting the Toronto archives and asking if they can help with providing the details relating to this address? - if their surname was indeed White then this will make things a little easier.
http://www.toronto.ca/archives/reference.htmI did have a quick look on the Canadian 1911 census but I was unable to search by address alone and even with the surname 'White' it retuned many thousands of options
Dufferin Street appears to be found in Ward 6 however, if this helps.
If you can trace your Grandfathers cousins (ie their exact names - I am assuming that they were born in the UK and that your Grandfather was UK born??) you may be able to find them on passenger lists and this will help to confirm if they were indeed named White. That will also give you a date of birth to search more specifically on. The lists may also give an onwards address, which may well match the one on the back of your postcard. There appears to be quite a mix of origins living in the area (e.g. Irish, English and Scottish, or 'Scotch' , as they put it) so worth doing a little more investigative work here if you can. The Scottish friends could well be neighbours but until you identify the cousins it could be an even trickier business identifying them!!
The dress certainly leans towards an Edwardian-ish time frame. Looking at the chap on the left, he almost looks as though he is wearing a'20's style cardigan and I think I can see the moustached chap on the right with some spats on his shoes, which were certainly in vogue between 1910-1930's. Overall, I sense late 1910's, but could be wrong as I am not an expert on this!!
PM me if you need any help with this and I will try to help where I can.
Best wishes
Flosse
