What does this mean as far as the heritage of this tender? Is it possible that that the plate is a reproduction and so the number is a non-rostered number? Are there any similar plates preserved that could be compared to this plate to determine authenticity?

I attached the photo of the plate from the T1 website that you had posted the link for earlier. If the plate data is inaccurate, is it possible that this tender came from another road or is it definitely a PRR tender? Are there any other locations on the tender or trucks that would have identifying indicia? If so, could the trucks have been swapped with another tender during the life of the tender? Very interesting. Thanks.

If the plate data is inaccurate, is it possible that this tender came from another road or is it definitely a PRR tender?

An update from Kurt Bell:

PRR Tender No. 6659 was among a series of 21000 gal. & 25000 gal. tenders built w/o engines,

Built new 11/16/1939 by Altoona Shops as part of lot 6057 which was 25 class 210F75a tenders.

Built with class 4F5T1 trucks with Timken bearings and Standard B Modified stoker where stoker engine was on the tender.

Wesley

Penn Central retired PRR 999401 (no PC number assigned) under AFE 7-2-99-777 during 1972. Per the retirement information, it was converted to MoW service during 4/59. To double-check, I reviewed the PC 12/31/75 MoW roster and did not find it listed under a PRR or PC#. I also checked the CR 1986 MoW roster to be sure, and it is not listed there either (as expected).

That said, I can add some mystery to the search because I happen to have a 3/4 rear view photo of PRR 999401 on 6/13/65 in reasonably fresh MoW yellow in service as a water tender. I am not sure of the location, but because of the catenary in the background, I can rule out Hagerstown. [emphasis added] Here is a shot of the tender at Hagertown https://conrailphotos.thecrhs.org/photo ... -7-24-1978 in 1978. I am also attaching a wider Harv Khan shot of the Hagertown tender from the same angle with the coaling dock prominent in the background. Although both tenders appear to be set up for the same use (note location of overflow pipe) and I am not an expert on PRR tenders, I am 99% sure the 999401 is not the tender at Hagerstown for the following reasons.

It was painted MoW yellow and lettered by PRR. Why would a successor railroad repaint it back to PRR and reletter it?
Note the equipment (?) door front/right on the 999401, but not present on the Hagerstown tender
Note the doghouse on the 999401, but not present on the Hagerstown tender
Note the right rear corner step is deformed on the 999401, but normally shaped here http://www.trainweb.org/wnyrhs/tenderFrame1Source1.htm
Note the brake wheel in the photo of the 6659 here http://www.trainweb.org/wnyrhs/tenderFrame1Source1.htm It is not present on the 999401

I suppose PRR or PC (or a private group?) could have changed the appearance of the tender (replacing step, adding brake wheel, removing doghouse, eliminating equipment door) after conversion to MoW and then repainted and relettered it back to PRR, but that would require a lot of money for a lowly MoW tender that remained stationary at a remote terminal. Considering the financial situation of PRR and later PC, I can't imagine that happening. The 999401 was also retired by PC after the bankruptcy when every dollar counted, so it is very likely it went to scrap to recover investment dollars.

That is a long way of saying, I think the starting point of the 999401 is incorrect for the tender at Hagerstown.