The TR503 and likely TR502 crystals were supplied by Colorado Crystals.? They might be ale to provide the needed ordering information.? Some years back, they still had a few 55 MHz crystals left when I contacted them; that's where I obtained mine.
I assume you have seen this comment.? The tuning procedure in the TR503 manual works very well using only the associated spectrum analyzer.
Regards,
Bruce, KG6OJI
ebrucehunter