Yes that is a lubrication hole and yes it will leak pressure, but it is a controlled leak.
I don't mess around taking time to bother with pressure/vac test on saws that old, I consider it a waste of time.
The saw is old and the seals are deteriorated already, at least to some degree. Just change the seals and be done with it.
I did seals on my R16 not too long ago and it like any other Partner I have done used standard metric seals. I looked and it seems I neglected to write down the seals I used on it, but I believe they used the same 15x26x7mm seals I do have written down for the P70 etc.