A very old apple tree - was it grafted?

I've been told that the land behind my house (and the neighbours) was planted with apple trees as part of the WW2 War effort. Unfortunately, most of these trees have died/fallen over and survive only as stumps. One of the trees is still alive and fruiting - not bad for an 80+ year old tree that receives no TLC whatsoever. 

This thread is not about that tree however - its about a stump that has a substantial side shoot. I thought this was a sucker off the root stock and have ignored it. This week, for the first time, it has flowered! In fact, these are the first apple flowers to open this year. 

All the trees that were here in 1980 were very tall,  so there doesn't seem to have been any attempt to graft them onto dwarfing rootstocks. 
Might they have been grafted for another reason? 

Needless to say that I shall be leaving it to see what happens. I like surprises.