"X-rated" Dl-701

I remember the roster at the back of John Rehor's history of the Nickle Plate as describing most of the NYCStL's 4-axle notch-nosed 1800 hp Alco roadswitchers as Dl-701, but marking some of the last as Dl-701X. I had long assumed that the X-suffix meant that these were RS-36 (carbody revisions, type E throttle) and the earlier ones RS-11, but I'm not sure. The matter came up on the New York Central forum (string: New York Central RS-36?); one contributor thought X and XA suffixes on Alco spec numbers meant "built with parts from trade-ins".

Anybody know (the full, un-cut, un-censored story on) how some Dl-701 got their X rating?

Believe the suffix was 'XA'-with the DL-701's being contructed against 2000hp passenger unit trades.

Herr Spreng--
Thanks!!!
Someone on the New York Central forum also suggested that the suffix might have something to do with use of parts from trade-in units.
---
(Bangs forehead: why didn't I check this earlier?) The "Extra 2200 South" roster of RS32/RS36 (issue 76: cover date July-September 1982: Lord knows when it actualy came out, since there's a phot in the issue dated 10 August 1982) doesn't mention the "X" suffix (the article says that the RS-36 "retained the Dl-701 spec number, but probably should be Dl-701A), but does record that NKP 865-875 were built with PA trade-ins, and that they re-used the GT-566 main generators from the PA units: certainly enough of a design modification to merit some sort of spec number suffix!)

New York Central RS-32 8035-8044 (the second NYC order) were also built with PA trade-ins and also re-used the GT-566 generators.

Another item the NKP RS36's used from the PA's was a large-capacity air compressor. Starting up a standing loco with zero pounds MR, it was amazing how little time it took to build pressure - far quicker than most locos.