Really old property marker pins
Hi Gorbo;
Here's the thing. You cannot rely upon any single piece of boundary evidence to make a determination on where a boundary is. Surveyors, when producing their opinion on a property's boundary, craw upon dozens of different sources to create that opinion. These sources include current and historical surveys of the property in question and of the neighbouring properties, official plans of subdivision, the location of survey bars on neighbouring properties and, of course, those on the subject property.
Although the bars are fairly difficult to move, and it is illegal to do so, we always assume they may have been. So typically a surveyor will only rely on them if their location can be verified by cross referencing it with an independent source such as the surveys listed above.
In your case I would suggest that your best route is to either get a new survey done, or get a boundary stakeout. The new survey is the more expensive option but will confirm the property's boundaries. the stakeout will simply mark one (or more) boundaries "on the ground" using small brightly coloured pins. Both options represent a surveyor's legal opinion on the location of the boundary.
I hope this helps.