Is tax free Lump Sum payment from UK Pension subject to US tax?

I am a UKC/USC resident in the UK for over 40 years. I am confused about treaties concerning double taxation of pensions. My pension was from Universities Superannuation Scheme (USS) and I had a 25% tax free lump sum payment at the time my pension started.

Grateful for any clarification as to whether the IRS would tax this and by how much. I am hoping that it isn’t taxable.

This question is better placed in the UK-US Tax forum. Maybe a Moderator would be kind enough to move it there.

https://talk.uk-yankee.com/index.php?board=11.0

Thanks. I re-posted in the correct forum.