Does Anybody Reconise This Overprint On Postal Stationery?

Is this like the "post office department- official use" overprints or is it something different? I presume the "section 435 1/2" it is referring to is a section in the post office guide?

It's a section in the Guide, possibly one which requires the use of a "mute" cancel on 3rd or 4th class mail, as it was called in that era. I found a reference in Google, but it serves only to help date the cancel:

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"But what about Sec. 435½ P.L.& R.?

If you have Sec. 435½ P.L.& R. marked on your envelope.....it's pre- October 1, 1932. I don't yet have a verifiable start date for this mandatory notation, but I'm working on it." [Postal Laws & Regulations]

Here's more than you'll ever want to (or care to) know about that section of P. L. & R.(Postal Laws and Regulations):

You have a locally precanceled enveloped. Collectable but not valuable.

It's not a precancel, it's simply a mute [machine] cancel, as required, and the rubber stamp indicates the allowable 3rd class rate of 1c per piece.