Yes this spot is graft. Appearance of foliage and stiff sharp needles say Bosnian pine to me. Being in EU may make somewhat more probable than in USA. If not mistaken believe Mugo has fairly soft needles.
After a visit to RHS Wisley (https://www.rhs.org.uk/gardens/wisley) I found the picture below, so it looks like it is a Austrian Black Pine grafted onto an old bonsai.