the tire is NOT a POLYGLAS, "POLYGLAS" was Goodyear's brand/registered trademark for a POLYester-cord / fiberGLASs belted tire

"BH" is the plant code for this particular tire, but it appears that, as tire manufacturing plants/equipment were sold/moved overseas from the US/Canada the existing plant code moved (or was recycled) with them & "BH" is currently in CHINA

during the 1971 calendar year, the DOT code was standardized to what is shown above, where:
BH = manufacturer plant code
UF = size - G60-15
LEF = type (construction & sidewall style)
056 & 097 = date (2-digit week *01 thru 52 * followed by 1-digit representing year)

this format was used for 3 decades until the year 2000 when the year code was increased to 2-digits, so the last digit represents 197?, 198?, or 199?

if the original poster would share more info as far as brand & name/style/tread it is possible that "tire" was only available in one of the possible decades

also to complicate matters, the plant codes have recently been updated to 3-digits & most of the on-line resources no longer show the codes & plants from past decades, only current ones