A couple of months ago My father gave me all of his firearms,
One of them is a pre-64 model 70 featherweight made in 1955.
The scope that's mounted on it is a SPI 6x32 model No. 566

The scope appears to be in excellent shape i'm guessing it was manufactured in the late 50s early 60s.
I would like to Leave this rifle scope combination just as it is, my concerns are that due to the age of the scope maybe all of the nitrogen has leaked out, is there anyway to check this before I go out into the field?
Also does SPl stand for Swiss Percision instruments?
Thank you for any information or input offered.

Swiss Precision Instruments was (is?) a distributor/importer of machine shop accessories like dial indicators and calipers as well as a whole host of other handy machining aids. They were bought out by MSC several years ago. They never manufactured rifle scopes or anything else. Must be a different SPI.

S.ucks P.itch I.t ?......

Thank you both for the replies, Huntsman, I hope that's not what it stands for I was kinda wanting to leave the scope on purely for sentimental reasons, pretty funny though.

My Dad sporterized a 1917 Enfield around 1958. The scope that he used was an SPI. It is still on the gun today. It is however in my brother's possession and I haven't seen it in years. If I remember correctly the scope crosshairs were slightly off center in the upper right quadrant. He killed a many hill Texas whitetails w/that old rifle. powdr

The first scope I ever bought was an SPI in 1965 a 4X. Don't remember what the initials stood for. I still have it on a 22 rifle that's not handy at the moment. It is still functional and reasonably clear. I couldn't afford the Weaver K4 at the time and mounted it on a M70 270Win and used it until about 1994 when I placed a Monarch 3X9 on it.
You could place it in the fridge and take it out and see if it fogs. Good luck on your heirloom. I have a very battered Win 69A that my dad bought for me in a garage 2nd hand store for $12 in about 1960 wouldn't part with it for any thing. Still shoots as well as my eyes will let me.

Blacktailer is correct....and

SPI is still a machine tool industry supplier

I believe that at the time I bought my SPI scope it was Scientific Precision Instruments not Swiss Precision Instruments. I think it was made in Japan at the time. I do not have this scope at hand at the moment. Mine has an always centered reticle