For your intended purpose, yes, you made a mistake.

Lots of folks still fire low number 1903's, &, if catastrophic failures occur, they are so rare most of us have never heard of them in our lifetime. I've seen or have reliable reports of several catastrophic failures due to bore obstruction, & I fear that more.

Proof loads are meaningless when it comes to low number receiver failures. Those receivers that have failed did so at random using standard loads. It just can't be predicted.

If I were a subsistence hunter & a low number 1903 was all I had to feed my family, I'd shoot it. But, I'm not. I shoot for a hobby, & losing a little money isn't going to hurt me. Life is too short, & emergency room visits are too expensive, for me to take a chance shooting a rifle that may be unsafe. If I had a (non-collectible) low number 1903, I'd sell it to some one who wants it for parts.

The decision is yours.

Neal